chore(requirements): update gunicorn to 19.7.1

Minor bug fixes, 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==3.6.2
 docker-py==1.10.6
-gunicorn==19.7.0
+gunicorn==19.7.1
 idna==2.5
 jmespath==0.9.2
 jsonfield==2.0.1