Bump django from 1.6.2 to 1.11.28 in /requirements

Bumps [django](https://github.com/django/django) from 1.6.2 to 1.11.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@1.6.2...1.11.28)

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

Author: dependabot[bot]

requirements/base.txt

@@ -1,5 +1,5 @@
 # Bleeding edge Django
-django==1.6.2
+django==1.11.28
 
 # Configuration
 django-configurations==0.8