Set up a rails app server

This includes:
- Install rbenv plus dependencies
- Add directories
- Add nginx configuration
- Add upstart job

The app setup is done in the [`puppet-limelight`](https://github.com/alphagov/puppet-limelight) repository. While this is nicely separated it does result in a lot of duplication with [`puppet-backdrop`](https://github.com/alphagov/puppet-backdrop).

What do people think about having a single puppet module called `puppet-pp-apps` or something similar that has defined types for creating rails and flask apps while sharing all the vhost and upstart stuff?