Update psycopg2 to 2.7.3.1

[tnir]

Update psycopg2 from 2.6.3 to 2.7.3.1 to support postgsql 10 (psycopg/psycopg2#489).

• https://pypi.python.org/pypi/psycopg2/2.6.1
• https://pypi.python.org/pypi/psycopg2/2.7.3.1

changelog between 2.6.3 and 2.7.3.1

• http://initd.org/psycopg/docs/news.html#what-s-new-in-psycopg-2-7-3-1

Closes #103

[tnir]

Installing packages works with this patch:

$ pip install -r requirements.txt
[...]
Collecting psycopg2==2.7.3.1 (from -r requirements.txt (line 4))
  Using cached psycopg2-2.7.3.1-cp36-cp36m-macosx_10_6_intel.macosx_10_9_intel.macosx_10_9_x86_64.macosx_10_10_intel.macosx_10_10_x86_64.whl
[...]
Installing collected packages: django, django-debug-toolbar, gunicorn, psycopg2, whitenoise
Successfully installed django-1.8.18 django-debug-toolbar-1.5 gunicorn-19.4.5 psycopg2-2.7.3.1 whitenoise-3.0