chore(requirements): update DRF to v3.6.3

See https://www.django-rest-framework.org/topics/release-notes/#363

Author: mboersma

rootfs/requirements.txt

@@ -4,7 +4,7 @@ Django==1.11
 django-auth-ldap==1.2.11
 django-cors-middleware==1.3.1
 django-guardian==1.4.8
-djangorestframework==3.6.2
+djangorestframework==3.6.3
 docker-py==1.10.6
 gunicorn==19.7.1
 idna==2.5