Default to `lucid` for debian family

We build packages on Ubuntu 10.04 (lucid), then test them on later versions.
However, not all new versions are tested; currently only Ubuntu, and up to
version 12.04. By defaulting to `lucid` we allow users to install on any
debian-family platform, and we print a log message (or have a resource show in
the output formatter).

Long term we should fix the issue (#460), where we have tester systems for
other Ubuntu versions - in particular Ubuntu 14.04 as it's an LTS.