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?

Also, this does not include proxying external paths to backdrop read and write.