RoR push rejected to Heroku "Could not detect rake tasks"-Twitter bootstrap

I’m trying to push some modifications of my Ruby on Rails web to Heroku but it says “push rejected”. The error comes after “Detecting rake tasks” and here’s the message:

remote:  !     Could not detect rake tasks
remote:  !     ensure you can run `$ bundle exec rake -P` against your app
remote:  !     and using the production group of your Gemfile.
remote:  !     rake aborted!
remote:  !     LoadError: cannot load such file -- sass
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `block in require'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:258:in `load_dependency'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/font-awesome-sass-5.0.13/lib/font-awesome-sass.rb:48:in `configure_sass'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/font-awesome-sass-5.0.13/lib/font-awesome-sass.rb:13:in `load!'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/font-awesome-sass-5.0.13/lib/font-awesome-sass.rb:78:in `<top (required)>'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/bundler-1.17.3/lib/bundler/runtime.rb:81:in `require'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/bundler-1.17.3/lib/bundler/runtime.rb:81:in `block (2 levels) in require'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/bundler-1.17.3/lib/bundler/runtime.rb:76:in `each'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/bundler-1.17.3/lib/bundler/runtime.rb:76:in `block in require'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/bundler-1.17.3/lib/bundler/runtime.rb:65:in `each'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/bundler-1.17.3/lib/bundler/runtime.rb:65:in `require'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/bundler-1.17.3/lib/bundler.rb:114:in `require'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/config/application.rb:9:in `<top (required)>'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/Rakefile:6:in `require'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/Rakefile:6:in `<top (required)>'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/rake-13.0.1/lib/rake/rake_module.rb:29:in `load'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/rake-13.0.1/lib/rake/rake_module.rb:29:in `load_rakefile'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/rake-13.0.1/lib/rake/application.rb:703:in `raw_load_rakefile'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/rake-13.0.1/lib/rake/application.rb:104:in `block in load_rakefile'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/rake-13.0.1/lib/rake/application.rb:186:in `standard_exception_handling'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/rake-13.0.1/lib/rake/application.rb:103:in `load_rakefile'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/rake-13.0.1/lib/rake/application.rb:82:in `block in run'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/rake-13.0.1/lib/rake/application.rb:186:in `standard_exception_handling'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/rake-13.0.1/lib/rake/application.rb:80:in `run'
remote:  !     /tmp/build_6bd938d1e4b5daac679d06adc0d16a99/vendor/bundle/ruby/2.3.0/gems/rake-13.0.1/exe/rake:27:in `<top (required)>'
remote:  !     vendor/bundle/bin/rake:29:in `load'
remote:  !     vendor/bundle/bin/rake:29:in `<main>'

Just in case it was my modifications’ fault, I deleted my local project, cloned it from heroku, changed just a comment and tried to push it. But the same error appeared. Which is weird, because my website is working ok in Heroku, but it gives an error if I try to upload the same code which is hosted there and working.

As the message error suggests, I also executed bundle exec rake -P RAILS_ENV=production and it gave me the following result with no error at all:

rake about
    environment
rake app:template
    environment
rake app:templates:copy
rake app:update
    update:configs
    update:bin
    update:upgrade_guide_info
rake app:update:bin
rake app:update:configs
rake app:update:upgrade_guide_info
rake assets:clean
    environment
rake assets:clobber
    environment
rake assets:environment
rake assets:precompile
    environment
    yarn:install
rake cache_digests:dependencies
    environment
rake cache_digests:nested_dependencies
    environment
rake cloudinary:sync_static
rake db:_dump
rake db:abort_if_pending_migrations
    environment
    load_config
rake db:charset
    environment
    load_config
rake db:check_protected_environments
    environment
    load_config
rake db:collation
    environment
    load_config
rake db:create
    load_config
rake db:create:all
    load_config
rake db:drop
    load_config
    check_protected_environments
rake db:drop:_unsafe
    load_config
rake db:drop:all
    load_config
    check_protected_environments
rake db:environment:set
    environment
    load_config
rake db:fixtures:identify
    environment
    load_config
rake db:fixtures:load
    environment
    load_config
rake db:forward
    environment
    load_config
rake db:load_config
rake db:migrate
    environment
    load_config
rake db:migrate:down
    environment
    load_config
rake db:migrate:redo
    environment
    load_config
rake db:migrate:reset
    db:drop
    db:create
    db:migrate
rake db:migrate:status
    environment
    load_config
rake db:migrate:up
    environment
    load_config
rake db:populate
    environment
rake db:purge
    load_config
    check_protected_environments
rake db:purge:all
    load_config
    check_protected_environments
rake db:reset
    db:drop
    db:setup
rake db:rollback
    environment
    load_config
rake db:schema:cache:clear
    environment
    load_config
rake db:schema:cache:dump
    environment
    load_config
rake db:schema:dump
    environment
    load_config
rake db:schema:load
    environment
    load_config
    check_protected_environments
rake db:schema:load_if_ruby
    db:create
    environment
rake db:seed
rake db:setup
    db:schema:load_if_ruby
    db:structure:load_if_sql
    seed
rake db:structure:dump
    environment
    load_config
rake db:structure:load
    environment
    load_config
    check_protected_environments
rake db:structure:load_if_sql
    db:create
    environment
rake db:test:load
    db:test:purge
rake db:test:load_schema
    db:test:purge
rake db:test:load_structure
    db:test:purge
rake db:test:prepare
    environment
    load_config
rake db:test:purge
    environment
    load_config
    check_protected_environments
rake db:version
    environment
    load_config
rake default
    test
rake dev:cache
rake environment
rake haml:erb2haml
rake initializers
    environment
rake log:clear
rake middleware
    environment
rake notes
rake notes:custom
rake notes:fixme
rake notes:optimize
rake notes:todo
rake paperclip:clean
    environment
rake paperclip:find_broken_attachments
    environment
rake paperclip:refresh
    paperclip:refresh:metadata
    paperclip:refresh:thumbnails
rake paperclip:refresh:fingerprints
    environment
rake paperclip:refresh:metadata
    environment
rake paperclip:refresh:missing_styles
    environment
rake paperclip:refresh:thumbnails
    environment
rake railties:install:migrations
    db:load_config
rake restart
rake routes
    environment
rake secret
rake stats
rake test
rake test:controllers
    test:prepare
rake test:db
    db:test:prepare
    test
rake test:functionals
    test:prepare
rake test:generators
    test:prepare
rake test:helpers
    test:prepare
rake test:integration
    test:prepare
rake test:jobs
    test:prepare
rake test:mailers
    test:prepare
rake test:models
    test:prepare
rake test:prepare
rake test:run
    test
rake test:system
    test:prepare
rake test:units
    test:prepare
rake time:zones
rake time:zones:all
rake time:zones:local
rake time:zones:us
rake tmp
rake tmp/cache
rake tmp/cache/assets
rake tmp/pids
rake tmp/sockets
rake tmp:cache:clear
rake tmp:clear
    tmp:cache:clear
    tmp:sockets:clear
rake tmp:create
    tmp/cache
    tmp/sockets
    tmp/pids
    tmp/cache/assets
rake tmp:pids:clear
rake tmp:sockets:clear
rake users:avisar_entrenadores_suscripcion
    environment
rake users:avisar_rutinas_vacias
    environment
rake users:eliminar_no_confirmados
    environment
rake yarn:install

Here’s my Rakefile:

require File.expand_path('../config/application', __FILE__)

KeepMeFit::Application.load_tasks

My Gemfile:

source 'https://rubygems.org'

ruby '2.3.3'

gem 'rails', '5.1.4'

group :development, :test do
    gem 'sqlite3'
    gem 'activemodel'
    gem 'json'
    gem 'rails_real_favicon'
    gem 'rspec-rails'
    gem 'guard-rspec'
    gem 'rb-notifu'
end

group :production do
    gem 'pg', '~> 0.18'
end

group :test do
    gem 'capybara'
    gem 'factory_girl_rails', '4.1.0'
    gem 'launchy'
end

group :development do
    gem 'annotate'
    gem 'better_errors'
    gem 'binding_of_caller'
    gem 'meta_request'
    gem 'certified', platforms: [:mingw, :mswin, :x64_mingw, :jruby]
end

group :assets do
  gem 'sass-rails'
  gem 'coffee-rails'
  gem 'coffee-script-source', '1.8.0'
  gem 'uglifier', '>= 1.0.3'
end

gem 'jquery-rails'
gem 'jquery-ui-rails'
gem 'bcrypt', '~> 3.1.12', platforms: [:ruby, :x64_mingw, :mingw]
gem 'puma'
gem 'haml'
gem 'haml-rails'
gem 'bootstrap-sass'
require 'rbconfig'
gem 'wdm', '>= 0.1.0' if RbConfig::CONFIG['target_os'] =~ /mswin|mingw/i
gem 'modernizr-rails'
gem 'sprockets-rails', :require => 'sprockets/railtie'
gem 'bootstrap-datepicker-rails'
gem 'tzinfo-data'
gem 'will_paginate', '~> 3.1.0'
gem 'bootstrap-will_paginate'
gem 'friendly_id'
gem 'gon'
gem 'cloudinary'
gem "omniauth-google-oauth2"
gem 'omniauth-facebook'
gem 'omniauth-twitter'
gem 'figaro'
gem 'font-awesome-sass', '~> 5.0.6'
gem 'audited'
gem 'recaptcha', require: 'recaptcha/rails'
gem "chartkick"
gem "select2-rails"
gem 'carrierwave'
gem 'braintree'
gem 'data-confirm-modal'
gem "paperclip", "~> 5.2.1"
gem 'aws-sdk', '< 3.0'
gem "remotipart"
gem 'cookies_eu'
gem 'rails_12factor'
gem 'countries'

And my Gemfile.lock:

GEM
  remote: https://rubygems.org/
  specs:
    actioncable (5.1.4)
      actionpack (= 5.1.4)
      nio4r (~> 2.0)
      websocket-driver (~> 0.6.1)
    actionmailer (5.1.4)
      actionpack (= 5.1.4)
      actionview (= 5.1.4)
      activejob (= 5.1.4)
      mail (~> 2.5, >= 2.5.4)
      rails-dom-testing (~> 2.0)
    actionpack (5.1.4)
      actionview (= 5.1.4)
      activesupport (= 5.1.4)
      rack (~> 2.0)
      rack-test (>= 0.6.3)
      rails-dom-testing (~> 2.0)
      rails-html-sanitizer (~> 1.0, >= 1.0.2)
    actionview (5.1.4)
      activesupport (= 5.1.4)
      builder (~> 3.1)
      erubi (~> 1.4)
      rails-dom-testing (~> 2.0)
      rails-html-sanitizer (~> 1.0, >= 1.0.3)
    activejob (5.1.4)
      activesupport (= 5.1.4)
      globalid (>= 0.3.6)
    activemodel (5.1.4)
      activesupport (= 5.1.4)
    activerecord (5.1.4)
      activemodel (= 5.1.4)
      activesupport (= 5.1.4)
      arel (~> 8.0)
    activesupport (5.1.4)
      concurrent-ruby (~> 1.0, >= 1.0.2)
      i18n (~> 0.7)
      minitest (~> 5.1)
      tzinfo (~> 1.1)
    addressable (2.5.2)
      public_suffix (>= 2.0.2, < 4.0)
    annotate (2.7.2)
      activerecord (>= 3.2, < 6.0)
      rake (>= 10.4, < 13.0)
    arel (8.0.0)
    audited (4.5.0)
      activerecord (>= 4.0, < 5.2)
    autoprefixer-rails (7.1.6)
      execjs
    aws-sdk (2.11.1)
      aws-sdk-resources (= 2.11.1)
    aws-sdk-core (2.11.1)
      aws-sigv4 (~> 1.0)
      jmespath (~> 1.0)
    aws-sdk-resources (2.11.1)
      aws-sdk-core (= 2.11.1)
    aws-sigv4 (1.0.2)
    aws_cf_signer (0.1.3)
    bcrypt (3.1.12-x86-mingw32)
    better_errors (2.4.0)
      coderay (>= 1.0.0)
      erubi (>= 1.0.0)
      rack (>= 0.9.0)
    binding_of_caller (0.7.3)
      debug_inspector (>= 0.0.1)
    bootstrap-datepicker-rails (1.7.1.1)
      railties (>= 3.0)
    bootstrap-sass (3.3.7)
      autoprefixer-rails (>= 5.2.1)
      sass (>= 3.3.4)
    bootstrap-will_paginate (1.0.0)
      will_paginate
    braintree (2.80.1)
      builder (>= 2.0.0)
    builder (3.2.3)
    callsite (0.0.11)
    capybara (2.15.4)
      addressable
      mini_mime (>= 0.1.3)
      nokogiri (>= 1.3.3)
      rack (>= 1.0.0)
      rack-test (>= 0.5.4)
      xpath (~> 2.0)
    carrierwave (1.2.2)
      activemodel (>= 4.0.0)
      activesupport (>= 4.0.0)
      mime-types (>= 1.16)
    certified (1.0.0)
    chartkick (2.2.5)
    climate_control (0.2.0)
    cloudinary (1.8.1)
      aws_cf_signer
      rest-client
    cocaine (0.5.8)
      climate_control (>= 0.0.3, < 1.0)
    coderay (1.1.2)
    coffee-rails (4.2.2)
      coffee-script (>= 2.2.0)
      railties (>= 4.0.0)
    coffee-script (2.4.1)
      coffee-script-source
      execjs
    coffee-script-source (1.8.0)
    concurrent-ruby (1.0.5)
    cookies_eu (1.7.1)
      js_cookie_rails (~> 2.2.0)
    countries (2.1.4)
      i18n_data (~> 0.8.0)
      money (~> 6.9)
      sixarm_ruby_unaccent (~> 1.1)
      unicode_utils (~> 1.4)
    country_select (3.1.1)
      countries (~> 2.0)
      sort_alphabetical (~> 1.0)
    crass (1.0.2)
    data-confirm-modal (1.6.1)
      railties (>= 3.0)
    debug_inspector (0.0.3)
    diff-lcs (1.3)
    domain_name (0.5.20170404)
      unf (>= 0.0.5, < 1.0.0)
    erubi (1.7.0)
    erubis (2.7.0)
    execjs (2.7.0)
    factory_girl (4.1.0)
      activesupport (>= 3.0.0)
    factory_girl_rails (4.1.0)
      factory_girl (~> 4.1.0)
      railties (>= 3.0.0)
    faraday (0.12.2)
      multipart-post (>= 1.2, < 3)
    ffi (1.9.18-x86-mingw32)
    figaro (1.1.1)
      thor (~> 0.14)
    font-awesome-sass (5.0.6)
      sass (>= 3.2)
    formatador (0.2.5)
    friendly_id (5.2.3)
      activerecord (>= 4.0.0)
    globalid (0.4.1)
      activesupport (>= 4.2.0)
    gon (6.2.0)
      actionpack (>= 3.0)
      multi_json
      request_store (>= 1.0)
    guard (2.14.1)
      formatador (>= 0.2.4)
      listen (>= 2.7, < 4.0)
      lumberjack (~> 1.0)
      nenv (~> 0.1)
      notiffany (~> 0.0)
      pry (>= 0.9.12)
      shellany (~> 0.0)
      thor (>= 0.18.1)
    guard-compat (1.2.1)
    guard-rspec (4.7.3)
      guard (~> 2.1)
      guard-compat (~> 1.1)
      rspec (>= 2.99.0, < 4.0)
    haml (5.0.4)
      temple (>= 0.8.0)
      tilt
    haml-rails (1.0.0)
      actionpack (>= 4.0.1)
      activesupport (>= 4.0.1)
      haml (>= 4.0.6, < 6.0)
      html2haml (>= 1.0.1)
      railties (>= 4.0.1)
    hashie (3.5.6)
    html2haml (2.2.0)
      erubis (~> 2.7.0)
      haml (>= 4.0, < 6)
      nokogiri (>= 1.6.0)
      ruby_parser (~> 3.5)
    http-cookie (1.0.3)
      domain_name (~> 0.5)
    i18n (0.9.0)
      concurrent-ruby (~> 1.0)
    i18n_data (0.8.0)
    jmespath (1.3.1)
    jquery-rails (4.3.1)
      rails-dom-testing (>= 1, < 3)
      railties (>= 4.2.0)
      thor (>= 0.14, < 2.0)
    jquery-ui-rails (6.0.1)
      railties (>= 3.2.16)
    js_cookie_rails (2.2.0)
      railties (>= 3.1)
    json (2.1.0)
    jwt (1.5.6)
    launchy (2.4.3)
      addressable (~> 2.3)
    listen (3.1.5)
      rb-fsevent (~> 0.9, >= 0.9.4)
      rb-inotify (~> 0.9, >= 0.9.7)
      ruby_dep (~> 1.2)
    loofah (2.1.1)
      crass (~> 1.0.2)
      nokogiri (>= 1.5.9)
    lumberjack (1.0.12)
    mail (2.7.0)
      mini_mime (>= 0.1.1)
    meta_request (0.4.3)
      callsite (~> 0.0, >= 0.0.11)
      rack-contrib (>= 1.1, < 3)
      railties (>= 3.0.0, < 5.2.0)
    method_source (0.9.0)
    mime-types (3.1)
      mime-types-data (~> 3.2015)
    mime-types-data (3.2016.0521)
    mimemagic (0.3.2)
    mini_mime (0.1.4)
    mini_portile2 (2.3.0)
    minitest (5.10.3)
    modernizr-rails (2.7.1)
    money (6.13.0)
      i18n (>= 0.6.4, <= 2)
    multi_json (1.12.2)
    multi_xml (0.6.0)
    multipart-post (2.0.0)
    nenv (0.3.0)
    netrc (0.11.0)
    nio4r (2.1.0)
    nokogiri (1.8.1-x86-mingw32)
      mini_portile2 (~> 2.3.0)
    notiffany (0.1.1)
      nenv (~> 0.1)
      shellany (~> 0.0)
    oauth (0.5.4)
    oauth2 (1.4.0)
      faraday (>= 0.8, < 0.13)
      jwt (~> 1.0)
      multi_json (~> 1.3)
      multi_xml (~> 0.5)
      rack (>= 1.2, < 3)
    omniauth (1.7.1)
      hashie (>= 3.4.6, < 3.6.0)
      rack (>= 1.6.2, < 3)
    omniauth-facebook (4.0.0)
      omniauth-oauth2 (~> 1.2)
    omniauth-google-oauth2 (0.2.10)
      addressable (~> 2.3)
      jwt (~> 1.0)
      multi_json (~> 1.3)
      omniauth (>= 1.1.1)
      omniauth-oauth2 (~> 1.3.1)
    omniauth-oauth (1.1.0)
      oauth
      omniauth (~> 1.0)
    omniauth-oauth2 (1.3.1)
      oauth2 (~> 1.0)
      omniauth (~> 1.2)
    omniauth-twitter (1.4.0)
      omniauth-oauth (~> 1.1)
      rack
    paperclip (5.2.1)
      activemodel (>= 4.2.0)
      activesupport (>= 4.2.0)
      cocaine (~> 0.5.5)
      mime-types
      mimemagic (~> 0.3.0)
    pg (0.21.0-x86-mingw32)
    pry (0.11.2)
      coderay (~> 1.1.0)
      method_source (~> 0.9.0)
    public_suffix (3.0.0)
    puma (3.11.0)
    rack (2.0.5)
    rack-contrib (1.2.0)
      rack (>= 0.9.1)
    rack-test (0.7.0)
      rack (>= 1.0, < 3)
    rails (5.1.4)
      actioncable (= 5.1.4)
      actionmailer (= 5.1.4)
      actionpack (= 5.1.4)
      actionview (= 5.1.4)
      activejob (= 5.1.4)
      activemodel (= 5.1.4)
      activerecord (= 5.1.4)
      activesupport (= 5.1.4)
      bundler (>= 1.3.0)
      railties (= 5.1.4)
      sprockets-rails (>= 2.0.0)
    rails-dom-testing (2.0.3)
      activesupport (>= 4.2.0)
      nokogiri (>= 1.6)
    rails-html-sanitizer (1.0.3)
      loofah (~> 2.0)
    rails_12factor (0.0.3)
      rails_serve_static_assets
      rails_stdout_logging
    rails_real_favicon (0.0.7)
      json (>= 1.7, < 3)
      rails (>= 3.1)
      rest-client (~> 2.0)
      rubyzip (~> 1)
    rails_serve_static_assets (0.0.5)
    rails_stdout_logging (0.0.5)
    railties (5.1.4)
      actionpack (= 5.1.4)
      activesupport (= 5.1.4)
      method_source
      rake (>= 0.8.7)
      thor (>= 0.18.1, < 2.0)
    rake (12.2.1)
    rb-fsevent (0.10.2)
    rb-inotify (0.9.10)
      ffi (>= 0.5.0, < 2)
    rb-notifu (0.0.4)
    recaptcha (4.6.2)
      json
    remotipart (1.4.0)
    request_store (1.3.2)
    rest-client (2.0.2-x86-mingw32)
      ffi (~> 1.9)
      http-cookie (>= 1.0.2, < 2.0)
      mime-types (>= 1.16, < 4.0)
      netrc (~> 0.8)
    rspec (3.7.0)
      rspec-core (~> 3.7.0)
      rspec-expectations (~> 3.7.0)
      rspec-mocks (~> 3.7.0)
    rspec-core (3.7.0)
      rspec-support (~> 3.7.0)
    rspec-expectations (3.7.0)
      diff-lcs (>= 1.2.0, < 2.0)
      rspec-support (~> 3.7.0)
    rspec-mocks (3.7.0)
      diff-lcs (>= 1.2.0, < 2.0)
      rspec-support (~> 3.7.0)
    rspec-rails (3.7.1)
      actionpack (>= 3.0)
      activesupport (>= 3.0)
      railties (>= 3.0)
      rspec-core (~> 3.7.0)
      rspec-expectations (~> 3.7.0)
      rspec-mocks (~> 3.7.0)
      rspec-support (~> 3.7.0)
    rspec-support (3.7.0)
    ruby_dep (1.5.0)
    ruby_parser (3.10.1)
      sexp_processor (~> 4.9)
    rubyzip (1.2.1)
    sass (3.5.3)
      sass-listen (~> 4.0.0)
    sass-listen (4.0.0)
      rb-fsevent (~> 0.9, >= 0.9.4)
      rb-inotify (~> 0.9, >= 0.9.7)
    sass-rails (5.0.6)
      railties (>= 4.0.0, < 6)
      sass (~> 3.1)
      sprockets (>= 2.8, < 4.0)
      sprockets-rails (>= 2.0, < 4.0)
      tilt (>= 1.1, < 3)
    select2-rails (4.0.3)
      thor (~> 0.14)
    sexp_processor (4.10.0)
    shellany (0.0.1)
    sixarm_ruby_unaccent (1.2.0)
    sort_alphabetical (1.1.0)
      unicode_utils (>= 1.2.2)
    sprockets (3.7.2)
      concurrent-ruby (~> 1.0)
      rack (> 1, < 3)
    sprockets-rails (3.2.1)
      actionpack (>= 4.0)
      activesupport (>= 4.0)
      sprockets (>= 3.0.0)
    sqlite3 (1.3.13-x86-mingw32)
    temple (0.8.0)
    thor (0.20.0)
    thread_safe (0.3.6)
    tilt (2.0.8)
    tzinfo (1.2.4)
      thread_safe (~> 0.1)
    tzinfo-data (1.2017.3)
      tzinfo (>= 1.0.0)
    uglifier (3.2.0)
      execjs (>= 0.3.0, < 3)
    unf (0.1.4)
      unf_ext
    unf_ext (0.0.7.4-x86-mingw32)
    unicode_utils (1.4.0)
    wdm (0.1.1)
    websocket-driver (0.6.5)
      websocket-extensions (>= 0.1.0)
    websocket-extensions (0.1.2)
    will_paginate (3.1.6)
    xpath (2.1.0)
      nokogiri (~> 1.3)

PLATFORMS
  x86-mingw32

DEPENDENCIES
  activemodel
  annotate
  audited
  aws-sdk (< 3.0)
  bcrypt (~> 3.1.12)
  better_errors
  binding_of_caller
  bootstrap-datepicker-rails
  bootstrap-sass
  bootstrap-will_paginate
  braintree
  capybara
  carrierwave
  certified
  chartkick
  cloudinary
  coffee-rails
  coffee-script-source (= 1.8.0)
  cookies_eu
  countries
  country_select (~> 3.1)
  data-confirm-modal
  factory_girl_rails (= 4.1.0)
  figaro
  font-awesome-sass (~> 5.0.6)
  friendly_id
  gon
  guard-rspec
  haml
  haml-rails
  jquery-rails
  jquery-ui-rails
  json
  launchy
  meta_request
  modernizr-rails
  omniauth-facebook
  omniauth-google-oauth2
  omniauth-twitter
  paperclip (~> 5.2.1)
  pg (~> 0.18)
  puma
  rails (= 5.1.4)
  rails_12factor
  rails_real_favicon
  rb-notifu
  recaptcha
  remotipart
  rspec-rails
  sass-rails
  select2-rails
  sprockets-rails
  sqlite3
  tzinfo-data
  uglifier (>= 1.0.3)
  wdm (>= 0.1.0)
  will_paginate (~> 3.1.0)

RUBY VERSION
   ruby 2.3.3p222

BUNDLED WITH
   1.16.1

EDIT

I add my application.scss (not complete because it exceeds the number of chars):

/*
 *= require_tree .
 *= require_self
 *= require bootstrap-datepicker
 *= require jquery-ui
 *= require select2
 *= require select2-bootstrap
 */
@import "bootstrap-sprockets";
@import "bootstrap";
//@import "bootstrap-responsive";
@import "welcome";
@import "users";
@import "sessions";
@import "extra_ejercicios";
@import "calendar";
@import "ejercicios";
@import "tablet";
@import "font-awesome-sprockets";
@import "font-awesome";
@import 'cookies_eu';
/* mixins, variables, etc. */

$grayMediumLight: #eaeaea;
$grayLighter: #fafafa;
$grayLight: #efefef;
$grayDarker: #303030;

@mixin box_sizing {
  -moz-box-sizing: border-box;
  -webkit-box-sizing: border-box;
  box-sizing: border-box;
}

/* universal */
html {
  overflow-y: scroll;
}

.main {
  width: 100%;
  padding-top: 20px;
}

.no-padding{
  padding: 0 !important;
}
.no-margin{
  margin: 0 !important;
}

textarea {
  resize: vertical;
}

.center {
  text-align: center;
}

.hidden{
  display: none;
}

.rutinas {
  padding: 0;
  list-style-type: none;
  margin: 0;
  border-bottom: 2px solid lightgray;
}
.rutinas-li{
  overflow: visible;
  padding: 10px 0;
  border-top: 2px solid lightgray;
}

.gravatar {
  @extend .img-rounded
}
...

And my application.js:

//= require jquery
//= require jquery_ujs
//= require jquery.remotipart
//= require jquery-ui
//= require_tree .
// Loads all Bootstrap javascripts
//= require bootstrap
//= require bootstrap-datepicker
//= require Chart.bundle
//= require chartkick
//= require select2
//= require data-confirm-modal
//= require cookies_eu

//This tells jQuery to send “text/javascript” in the request headers with every request (GET, POST, etc.).
//Rails will recognize this as a request for JSON and will respond accordingly.
jQuery.ajaxSetup({
    'beforeSend': function(xhr) { xhr.setRequestHeader("Accept", "text/javascript") }
});

//http://samuelmullen.com/2011/02/dynamic-dropdowns-with-rails-jquery-and-ajax/
$.fn.subSelectWithAjax = function() {
    var that = this;

    this.change(function() {
        $.post(that.attr('rel'), {id: that.val()}, null, "script");
    });
};

$.fn.checkBoxAjax = function() {
    var that = this;

    this.change(function() {
        $.post(that.attr('rel'), {checked: that.is(':checked')}, null, "script");
    });
};

$.fn.verRutinaAjax = function() {
    var that = this;

    this.click(function(){
        $('.clicked').removeClass('clicked');
        $(this).addClass('clicked');
        var id = $(this).attr('usrid');
        $.post(that.attr('rel'), {id: id}, null, "script");
    });
};