upgrade to current postgres client library

requirements/production.txt

@@ -2,7 +2,7 @@
 # Additional dependencies for Heroku deployment
 dj-database-url==0.4.1
 uwsgi==2.0.14
-psycopg2==2.6.2
+psycopg2==2.7.3.1
 whitenoise==3.2.2
 boto==2.45.0
 django-storages==1.5.2