Rails 5 requires explicit rails gem

```
Bundler could not find compatible versions for gem "activemodel":

In Gemfile:
  activerecord was resolved to 5.2.0.alpha, which depends on activemodel (= 5.2.0.alpha)

  rails (>= 3.0) was resolved to 4.2.9, which depends on activemodel (= 4.2.9)
```

Gemfile

@@ -8,6 +8,7 @@ version = ENV['RAILS_VERSION'] || '4.0'
 if version == 'master'
   gem 'rack', github: 'rack/rack'
   gem 'arel', github: 'rails/arel'
+  gem 'rails', github: 'rails/rails'
   git 'https://github.com/rails/rails.git' do
     gem 'railties'
     gem 'activesupport'