Bump django from 1.11.21 to 1.11.28 in /sandbox

Bumps [django](https://github.com/django/django) from 1.11.21 to 1.11.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@1.11.21...1.11.28)

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

sandbox/requirements.txt

@@ -1,5 +1,5 @@
 boto==2.40.0
-Django==1.11.21
+Django==1.11.28
 django-simple-pagination==1.1.4
 django-storages==1.4.1
 Pillow==4.3.0