chore(requirements): update docker-py to 1.8.1

https://github.com/docker/docker-py/blob/master/docs/change_log.md#181

Author: helgi

rootfs/requirements.txt

@@ -4,7 +4,7 @@ Django==1.9.5
 django-cors-headers==1.1.0
 django-guardian==1.4.4
 djangorestframework==3.3.3
-docker-py==1.8.0
+docker-py==1.8.1
 gunicorn==19.4.5
 jsonfield==1.0.3
 morph==0.1.2