Deprecate this repository

It has been replaced by https://github.com/thoughtbot/homebrew-formulae

I'm hoping removing the forumla from here will prevent Homebrew from getting confused when users tap the new repo and it contains the same formula that this one did.

See also:
* https://github.com/thoughtbot/homebrew-formulae/pull/1
* https://github.com/thoughtbot/gitsh/pull/59
* https://github.com/thoughtbot/rcm/pull/25