have github locations inherit the to_s of git locations

This pull request ~~addresses issue #845~~ [edited to reflect that this should be done independently of issue #845] improves github locations by having them use the `to_s` inherited by the `GitLocation` super class.  This would make an accidental extra `.git` clear because the new display looks like the following, for example:

    Fetching ... from github: 'git://github.com/RiotGames/berkshelf-cookbook-fixture.git.git' ...

In addition, the code is more DRY, we have consistency in presentation between git and github locations, and github locations now display more information (by including the ref as git locations already do).

I updated the tests and added some new ones.