chore(deps): bump django from 1.11.20 to 1.11.21 in /rootfs

Bumps [django](https://github.com/django/django) from 1.11.20 to 1.11.21.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@1.11.20...1.11.21)

Signed-off-by: dependabot[bot] <support@github.com>

rootfs/requirements.txt

@@ -1,6 +1,6 @@
 # Deis controller requirements
 backoff==1.4.3
-django==1.11.20
+django==1.11.21
 django-auth-ldap==1.2.15
 django-cors-middleware==1.3.1
 django-guardian==1.4.9