[feature] List reverse dependencies for a specific gem

I implemented some new API methods to add the ability for reverse dependency fetching. Specifically, I have added a model helper called "rdepends" (in both 'Version' and 'Rubygem' models) to query the database for all the reverse dependencies of a specific gem. The helpers are exposed through API methods in `api/v1/{version,rubygems}` controllers. I have also included tests covering the new code pieces.

My idea is to implement (in both server and client side) a functionality similar to `apt-rdepends -r` utility. A user will be able to provide a name of a gem and the rubygems will return a list of all the reverse dependencies related to the name given. So, the next step would be to implement the client side part in rubygems library (or in bundler gem).