Allow rbtools to be used from anywhere

Testing Done:
I used this to post https://rbcommons.com/s/twitter/r/1412, because I didn't have another handy way to use rbtools.

Reviewed at https://rbcommons.com/s/twitter/r/1413/
Sync of https://github.com/pantsbuild/pants/commit/0e3dd4e28e82a6aa5c50ce270221c7e7b759829e