When the example app is generated it puts a Rails version that is defined by currently installed version. This works great in most situtations, except `master` or other edge branches. The version listed is the current version of Rails, so they diverge. Move how we load rails into a custom Gemfile, which we eval ourselves. We then remove the rails version put into the gemfile and share our custom version. Due to when `rake` and other shell processes fork, the RAILS_VERSION environment variable is properly passed through.