Bump django from 2.1.10 to 2.1.11

Bumps [django](https://github.com/django/django) from 2.1.10 to 2.1.11.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.1.10...2.1.11)

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

pip-freeze.txt

@@ -25,7 +25,7 @@ django-redis==4.10.0
 django-rosetta==0.9.0
 django-storages==1.7.1
 django-timezone-field==3.0  # via rapidpro-dash
-django==2.1.10
+django==2.1.11
 djangorestframework==3.10.1
 flake8==3.6.0
 gunicorn==19.9.0