tests: correct tox env django spec for ver 1.11
tests: correct tox env django spec for ver 1.11

dsanders11/django-advanced-filters,modlinltd/django-advanced-filters,dsanders11/django-advanced-filters,modlinltd/django-advanced-filters,dsanders11/django-advanced-filters,dsanders11/django-advanced-filters,modlinltd/django-advanced-filters