chore(requirements): update django-guardian to 1.4.9

See https://github.com/django-guardian/django-guardian/blob/devel/CHANGES#release-149-jul-01-2017

Author: mboersma

rootfs/requirements.txt

@@ -3,7 +3,7 @@ backoff==1.4.3
 Django==1.11.3
 django-auth-ldap==1.2.14
 django-cors-middleware==1.3.1
-django-guardian==1.4.8
+django-guardian==1.4.9
 djangorestframework==3.6.3
 docker-py==1.10.6
 gunicorn==19.7.1