psycopg 2.8 is now out, which means that the C library gets built from source,
so we now need libpq-dev when building.

Turns out the need for this package is already documented in
docs/postgres.rst.

Make sure it refreshes the apt cache before trying to install stuff

since #4298, the optional dependencies are no longer installed with a simple
`pip install .`, which meant that they were not being included in the debian
package.

The easy fix to that is dh_virtualenv --extras, but that needs dh_virtualenv
1.1...