Don't require psycopg2 to operate.

Jeepyb optionally supports postgres via psycopg2 but it isn't required
for base operation. Remove the requirement so that jeepyb can be
installed on existing jeepyb installs without needing to add libpq-dev
everywhere.

Users wishing to use postgres with jeepyb should install psycopg2 in
addition to jeepyb.

Change-Id: Ibe383a27bf826af02b9ad1764d224d654d195519