fixed very evil 'tip' tag oversight in bitbucket driver

The HgDriver already does the right thing, I suppose someone (me?) just didn't think of tip in the BitbucketDriver. This fixes composer/packagist#285 as well.

If possible, I'd like to see this deployed ASAP, but it seems for that we need to update packagist to Symfony 2.3 (as Composer's dev-master requires 2.3). I can PR a fix for that as well, but I can't really test the whole application.