Use gzip compression in Ubuntu packages to make it possible to install on Lucid

Lucid comes with an older version of dpkg which does not support xz compression. Much of the work done to make Docker compatible with RHEL 6 has also had the side effect of making it runnable on Ubuntu 10.04 Lucid.

This change makes the .deb files installable on Lucid. Gzip-compressed debs [should continue to work for future Ubuntu/Debian versions for a long time](https://botbot.me/freenode/docker-dev/msg/8532969)