Address issue #845 by having github locations use git locations's to_s

I addressed this issue by having github locations use the `to_s` inherited by the GitLocation super class.  This makes the 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.