Remove python-psycopg2 and add note about libpq-dev dependency.
Remove python-psycopg2 and add note about libpq-dev dependency.

python-psycopg2 is superseded by us having psycopg2 in requirements.txt anyway, but that package requires the postgres dev headers supplied by libpq-dev, so that it can be compiled when we pip install it.

mysociety/django-jumpstart,mysociety/django-jumpstart