Fix dependency for Gotham nightly

`apt-get install xbian-package-xbmc-gotham-nightly` produces the following:

`dpkg: xbian-package-xbmc: dependency problems, but removing anyway as you requested:
 xbian-package-upstart-xbmc-bridge depends on xbian-package-xbmc.
 xbian-update depends on xbian-package-xbmc | xbian-package-xbmc-6q; however:
  Package xbian-package-xbmc is to be removed.
  Package xbian-package-xbmc-6q is not installed.`

I'm not sure if xbian-package-xbmc-6q still exists, as I can't find it in the repository, however I didn't remove it.