Replace double quotes with single quotes

Gemfile

@@ -6,7 +6,7 @@ gem 'arel', github: 'rails/arel', branch: 'master'
 
 gem 'mocha', '>= 0.11.2', :require => false
 gem 'rack-test', github: 'brynary/rack-test'
-gem 'rack-cache', "~> 1.2"
+gem 'rack-cache', '~> 1.2'
 gem 'bcrypt-ruby', '~> 3.0.0'
 gem 'jquery-rails', '~> 2.1.4', github: 'rails/jquery-rails'
 gem 'turbolinks'