Remove explicit dependency on rack for our build

Using rack from git opens up opportunities to fail due to unreleased
rack issues, e.g. rack/rack#1768

No need to explicitly depend on rack, Rails does that https://github.com/rails/rails/blob/c2b083df913f44e664576aafe24c41ec657f8eb5/actionpack/actionpack.gemspec#L37

Author: pirj

Gemfile-rails-dependencies

@@ -8,7 +8,6 @@ when /master/
   gem "activerecord-deprecated_finders", :git => "https://github.com/rails/activerecord-deprecated_finders.git"
   gem "web-console", :git => "https://github.com/rails/web-console", :group => :development
   gem 'coffee-rails', :git => "https://github.com/rails/coffee-rails.git"
-  gem 'rack', :git => 'https://github.com/rack/rack.git'
   gem 'i18n', :git => 'https://github.com/svenfuchs/i18n.git', :branch => 'master'
   gem 'sprockets', :git => 'https://github.com/rails/sprockets.git', :branch => 'master'
   gem 'sprockets-rails', :git => 'https://github.com/rails/sprockets-rails.git', :branch => 'master'