Support for Rails API applications

[robdimsdale]

The Rails Assets buildpack participates when there is a rails application with assets. Rails API applications do not have assets, and hence that buildpack does not participate.

But, that buildpack provides features (like setting RAILS_ENV) that would be useful/necessary in a Rails API application, too. We should evaluate creating a separate buildpack for Rails apps, and having the existing rails-assets buildpack focus soly on assets.