chore(requirements): update gunicorn to 19.7.0

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

Author: mboersma

rootfs/requirements.txt

@@ -6,7 +6,7 @@ django-cors-middleware==1.3.1
 django-guardian==1.4.6
 djangorestframework==3.5.4
 docker-py==1.10.6
-gunicorn==19.6.0
+gunicorn==19.7.0
 idna==2.4
 jmespath==0.9.1
 jsonfield==2.0.0