Stop building for squeeze, lucid, and raring

- Lucid Desktop is dead.
- Raring died in January 2014
- Squeeze will be EOL in June, but stop building now, because
  we don't care, and we're losing on Python versions.
Add the PPA on Ubuntu, always:

Per discussion in (Trac: #1377), we should always add the PPA on
Ubuntu, otherwise things will break.  The user can opt-out of it,
unless they're installing cluster.