Rails 4.0 is going to depend on Rack 1.5.x so let's test against Rack master

Gemfile

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