testing: remove django-discover-runner
testing: remove django-discover-runner

project supports Django 1.11+

> https://pypi.org/project/django-discover-runner/

This runner has been added to Django 1.6 as the default test runner.
If you use Django 1.6 or above you don’t need this app.

fix #250

jezdez/django-configurations,jazzband/django-configurations,jazzband/django-configurations