Remove rack-test git dependency

The dependency was introduced in this commit:

002713c6

which has been released a long time ago

Gemfile

@@ -5,7 +5,6 @@ gemspec
 gem 'arel', github: 'rails/arel', branch: 'master'
 
 gem 'mocha', '~> 0.13.0', require: false
-gem 'rack-test', github: 'brynary/rack-test'
 gem 'rack-cache', '~> 1.2'
 gem 'bcrypt-ruby', '~> 3.0.0'
 gem 'jquery-rails', '~> 2.2.0', github: 'rails/jquery-rails'