Make the tests pass on the Rails master

Rails 5 does not include `assert_template` but we need this on the tests of
the helper.

This add the `rails-controller-testing` to keep support on `assert_template`.

Gemfile

@@ -12,6 +12,7 @@ version = ENV['RAILS_VERSION'] || '4.2'
 if version == 'master'
   gem 'rack', github: 'rack/rack'
   gem 'arel', github: 'rails/arel'
+  gem 'rails-controller-testing', github: 'rails/rails-controller-testing'
   git 'https://github.com/rails/rails.git' do
     gem 'railties'
     gem 'activesupport'

test/active_model_serializers/test/serializer_test.rb

@@ -1,4 +1,5 @@
 require 'test_helper'
+require 'rails-controller-testing' if Rails::VERSION::MAJOR >= 5
 
 module ActiveModelSerializers
   module Test