[Security] Bump django from 1.9 to 1.11.28

Bumps [django](https://github.com/django/django) from 1.9 to 1.11.28. **This update includes security fixes.**
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@1.9...1.11.28)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -4,7 +4,7 @@ beautifulsoup4==4.4.1
 billiard==3.3.0.22
 celery==3.1.19
 dj-database-url==0.3.0
-Django==1.9
+Django==1.11.28
 django-absoluteuri==1.1.0
 django-appconf==1.0.1
 django-bootstrap-form==3.2