remove mongodb package dependency

It is not a package dependency.
Mongodb could be running on a different server and is not a required
package for this package to function.

Also along for the ride in this PR:
* use 3.0 (native) package format so that source packages are xz compressed instead of tar.gz.
* target trusty by default when updating the changelog using dch
Github apparently won't let me put a comment on line 47, but it appears to be out of date.