I'm doing the Rails Tutorial by Michael Hartl and I ran into this problem when trying to run the rails server.
This is is my gem file
gem 'rails', '4.1.5'
gem 'sqlite3'
gem 'sass-rails', '~> 4.0.3'
gem 'uglifier', '>= 1.3.0
gem 'coffee-rails', '~> 4.0.0'
gem 'jquery-rails'
gem 'turbolinks'
gem 'jbuilder', '~> 2.0'
gem 'sdoc', '~> 0.4.0',
gem 'tzinfo-data', platforms: [:mingw, :mswin]