Remove a mention that a metric only works with Django 1.8.
Remove a mention that a metric only works with Django 1.8.

We don't support earlier versions of Django.

korfuri/django-prometheus,wangwanzhong/django-prometheus,korfuri/django-prometheus,obytes/django-prometheus,wangwanzhong/django-prometheus,DingaGa/django-prometheus,obytes/django-prometheus,DingaGa/django-prometheus