Weird message in my terminal since Rails 5 upgrade – Twitter bootstrap

Today I’ve upgraded my system to rails 5 for a special project.
I am back on another project on rails 4 and I have troubles…
for exemple here I am trying to run rails g rails_admin:install
My terminal isn’t happy with this…

$ rails g rails_admin:install
Could not find aws-sdk-core-2.5.7 in any of the sources
Run `bundle install` to install missing gems.

I ran bundle install and it keep moaning…
I tried bundle update still impossible
What should I do please ?

this is my gemfile

gem 'high_voltage', '~> 3.0.0'
gem 'slim', '~> 3.0', '>= 3.0.7'
gem 'mail_form', '~> 1.5', '>= 1.5.1'
gem 'simple_form', '~> 3.2', '>= 3.2.1'
gem 'masonry-rails', '~> 0.2.4'
gem 'carrierwave'
gem 'mini_magick'
gem 'fog'
gem 'aws-sdk', '~> 2'
gem 'rails', '4.2.5'
gem "twitter-bootstrap-rails"
gem 'bootstrap-sass', '~> 3.3.6'
gem 'sass-rails', '>= 3.2'
gem 'font-awesome-rails', '~> 4.6', '>= 4.6.3.0'
gem 'uglifier', '>= 1.3.0'
gem 'coffee-rails', '~> 4.1.0'
gem 'jquery-rails'
gem 'turbolinks'
gem 'jbuilder', '~> 2.0'
gem 'sdoc', '~> 0.4.0', group: :doc
gem 'devise'
gem 'pry', '~> 0.10.4'
gem 'rails_admin'
group :production do
    gem 'pg'
    gem 'rails_12factor'
end

group :development, :test do
  gem 'sqlite3',     '1.3.9'
  gem 'byebug',      '3.4.0'
  gem 'web-console', '2.0.0.beta3'
  gem 'spring',      '1.1.3'
  gem 'dotenv-rails', :groups => [:development, :test]
end

regards