add psycopg2 to requirements

[skorasaurus]

need to add to the requirements.txt
psycopg2

[skorasaurus]

when running for the first time as of eae9331; you'd get an error stating psycopg2 would need to be installed; (full output here -https://gist.github.com/skorasaurus/eb6f793d2cf22f4ea63fded53342bff9)

however, after I added psycopg2 to the requirements (be192c2#diff-b4ef698db8ca845e5845c4618278f29a) and ran pip -r requirements.txt ; I got a new prompt stating giving this error

(I pegged the version requirement from https://github.com/datamade/large-lots/commit/f4050da2e8bbc0e61d8b486d287f3c4274496d18)

Collecting psycopg2==2.6.2 (from -r requirements.txt (line 9))
Downloading psycopg2-2.6.2.tar.gz (376kB)
100% |████████████████████████████████| 378kB 404kB/s
Complete output from command python setup.py egg_info:
running egg_info
creating pip-egg-info/psycopg2.egg-info
writing pip-egg-info/psycopg2.egg-info/PKG-INFO
writing top-level names to pip-egg-info/psycopg2.egg-info/top_level.txt
writing dependency_links to pip-egg-info/psycopg2.egg-info/dependency_links.txt
writing manifest file 'pip-egg-info/psycopg2.egg-info/SOURCES.txt'
Error: You need to install postgresql-server-dev-X.Y for building a server-side extension or libpq-dev for building a client-side application.
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-zhSnvr/psycopg2/

View code again to see why psycopg2 is needed; it's possible that since it wasn't originally earlier ; that an earlier commit introduced this requirement.

(additional notes; on my work computer, 16.04 ubuntu, I did sudo apt-get install libpq-dev ; installed that packaged; pip -r requirements.txt again and got a different error: https://gist.github.com/5778f86deab0231ae59375c028107082