Лучше обучайся по свежей книжке
https://www.railstutorial.org/book source 'https://rubygems.org'
gem 'rails', '4.2.1'
gem 'sass', '3.4.13'
gem 'sass-rails', '5.0.3'
gem 'uglifier', '2.7.1'
gem 'coffee-rails', '4.1.0'
gem 'jquery-rails', '4.0.3'
gem 'turbolinks', '2.5.3'
gem 'jbuilder', '2.2.13'
gem 'sdoc', '0.4.1', group: :doc
group :development, :test do
gem 'sqlite3', '1.3.10'
gem 'byebug', '4.0.5'
gem 'web-console', '2.1.2'
gem 'spring', '1.3.4'
end
group :production do
gem 'pg', '0.17.1'
gem 'rails_12factor', '0.0.2'
end