Bump django from 1.11.21 to 1.11.28 in /openedx_caliper_tracking/tests

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>

openedx_caliper_tracking/tests/test_requirements.txt

@@ -1,5 +1,5 @@
 celery==4.3.0
-Django==1.11.21
+Django==1.11.28
 isodate==0.6.0
 mock==3.0.5
 python-dateutil==2.8.0