Reduce Buildpack strictness from "Rails" to "Rails-compatible"

This does a couple of things:

* Makes the assumption that if the Rails-like files are there, then the app is Rails-y enough. This allows us to run Goliath or Sinatra apps so long as they follow the same conventions. (Previous assumption was `config/environment.rb`, but nothing else in the script actually uses it. This now checks for `config/database.yml` and `config/boot.rb` instead.)
* Runs Asset Precompilation only if the task is actually present. [Rails-API](https://github.com/rails-api/rails-api) apps aren't going to have the task at all, for example.

This is currently running on a test instance; I've pinged you the app name so you can check out the logs if you need to.