chore(django): upgrade to django version 1.11.20 latest patch

There is a new security patch released recently for django.

Author: Cryptophobia

rootfs/requirements.txt

@@ -1,6 +1,6 @@
 # Deis controller requirements
 backoff==1.4.3
-django==1.11.18
+django==1.11.20
 django-auth-ldap==1.2.15
 django-cors-middleware==1.3.1
 django-guardian==1.4.9