Remove django<1.8 from tests
Remove django<1.8 from tests

Django < 1.8 is not supported any more by upstream, and Horizon
has dropped django 1.7 and lower version support, so we don't need
to keep them here.

Change-Id: Ic4c1500c3403652f5e0936d81ad2797f1ab4b368

openstack/mistral-dashboard,openstack/mistral-dashboard,openstack/mistral-dashboard