Bump django from 1.11.4 to 1.11.28

Bumps [django](https://github.com/django/django) from 1.11.4 to 1.11.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@1.11.4...1.11.28)

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

requirements.txt

@@ -12,7 +12,7 @@ django-tinymce==2.6.0
 django-webtest==1.9.2
 django-widget-tweaks==1.4.1
 django-mptt==0.8.7
-Django==1.11.4
+Django==1.11.28
 django-transmeta-edw==0.7.3
 graypy==0.2.14
 gunicorn==19.7.1