chore(controller): update gunicorn to 19.3.0

See http://docs.gunicorn.org/en/19.3/news.html#id1

Author: mboersma

requirements.txt

@@ -11,7 +11,7 @@ django-guardian==1.2.5
 django-json-field==0.5.7
 djangorestframework==3.0.5
 docker-py==0.7.2
-gunicorn==19.2.1
+gunicorn==19.3.0
 paramiko==1.15.2
 psycopg2==2.6
 python-etcd==0.3.2