fix(extras): Simplify how we find extras package

- Current regexp breaks on kernels with double-digit versions (i.e.
  3.11.0-13-generic)
- Can't find a sensible reason why we can't plug in uname -r directly

This is tested working on Ubuntu Precise 12.04 and tested in theory on Ubuntu Saucy 13.10 (although other problems prevent this cookbook from working there yet; see my recently filed issue).