Clarify AMS dependencies

Gemfile

@@ -3,17 +3,26 @@ source 'https://rubygems.org'
 # Specify your gem's dependencies in active_model_serializers.gemspec
 gemspec
 
-gem "minitest"
+version = ENV['RAILS_VERSION'] || '4.2'
 
-version = ENV["RAILS_VERSION"] || "4.2"
-
-if version == "master"
-  gem "rails", github: "rails/rails"
-
-  # ugh https://github.com/rails/rails/issues/16063#issuecomment-48090125
-  gem "arel", github: "rails/arel"
+if version == 'master'
+  gem 'rack', github: 'rack/rack'
+  git 'https://github.com/rails/rails.git' do
+    gem 'railties'
+    gem 'activesupport'
+    gem 'activemodel'
+    gem 'actionpack'
+    # Rails 5
+    gem 'actionview'
+  end
+  # Rails 5
+  gem 'rails-controller-testing', github: 'rails/rails-controller-testing'
 else
-  gem "rails", "~> #{version}.0"
+  gem_version = "~> #{version}.0"
+  gem 'railties', gem_version
+  gem 'activesupport', gem_version
+  gem 'activemodel', gem_version
+  gem 'actionpack', gem_version
 end
 
 # Windows does not include zoneinfo files, so bundle the tzinfo-data gem