raring/bumblebee: Workaround for X.org bug

There is a bug in upstream X.org, that is uncovered in Ubuntu 13.04. It prevents Bumblebee from running anything on the discrete card unless BusID is set in xorg.conf files.

This closes https://github.com/Bumblebee-Project/Bumblebee/issues/367 for now on, still waiting for a proper fix upstream.