filter_for_reverse_field has been removed in django-filter==2.0.0

[GeyseR]

GrapheneFilterSetMixin should be updated properly for working with newer versions on django-filter

[jayhale]

I have a fork that seems to work with Django >=2 and django_filter >=2. The change just amounts to removing the reference to filter_for_reverse_field since filter_for_field handles all cases now.

master...jayhale:django-filter-2

Work needed for a PR:

• Update requirements to match Django >=1.11 and django-filter >=2
• Update requirements in examples
• Update docs