Bump django from 1.11.1 to 1.11.28 in /requirements

Bumps [django](https://github.com/django/django) from 1.11.1 to 1.11.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@1.11.1...1.11.28)

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

requirements/base.txt

@@ -4,7 +4,7 @@
 
 pyprind==2.9.1
 psycopg2==2.7.3.2
-Django==1.11.1
+Django==1.11.28
 Jinja2==2.7.3
 MarkupSafe==0.23
 Markdown==2.6.2