Add requires matching gem name.

Normally gem's tend to have a file/code structure matching their name, hopefully following the [naming conventions](http://guides.rubygems.org/name-your-gem/), but puffing billy doesn't. In the event a gem's structure doesn't match it's requires you will often find files matching the name (or sometimes misspellings (e.g. - for _)).

This PR adds the 'expected' require(s) given the gem name, in the hope of reducing "surprise" :)