Use Rack::Cache middleware only if is in Gemfile

Gemfile

@@ -6,6 +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 'bcrypt-ruby', '~> 3.0.0'
 gem 'jquery-rails'
 

actionpack/actionpack.gemspec

@@ -18,7 +18,6 @@ Gem::Specification.new do |s|
   s.requirements << 'none'
 
   s.add_dependency('activesupport', version)
-  s.add_dependency('rack-cache',    '~> 1.2')
   s.add_dependency('builder',       '~> 3.1.0')
   s.add_dependency('rack',          '~> 1.4.1')
   s.add_dependency('rack-test',     '~> 0.6.1')

railties/lib/rails/application.rb

@@ -282,6 +282,12 @@ module Rails
       ActionDispatch::MiddlewareStack.new.tap do |middleware|
         app = self
         if rack_cache = config.action_controller.perform_caching && config.action_dispatch.rack_cache
+          begin
+            require 'rack/cache'
+          rescue LoadError => error
+            error.message << ' Be sure to add rack-cache to your Gemfile'
+            raise
+          end
           require "action_dispatch/http/rack_cache"
           middleware.use ::Rack::Cache, rack_cache
         end