RFC: Spawn a secrets.yml that reads from the env

*This commit hasn't been tested, it's just for comment*

Having .yml files containing secrets is not very [12factor](http://12factor.net/config).  This commit makes the buildpack spawn a `config/secrets.yml` file as [expected by Rails 4.1](https://github.com/rails/rails/blob/master/guides/source/4_1_release_notes.md#configsecretsyml) applications containing everything that's in the environment.

This means that a Heroku Rails developer can still stipulate their configuration in the environment as per 12factor, but still use the `Rails.application.secrets.` paradigm to access 'secrets'.