djangorestframework is not a test_require

I though in #5 DRF was only used for testing but it's used in the very core of ROA for parsers/renderers like in https://github.com/charles-vdulac/django-roa/blob/master/django_roa/db/models.py#L25
Merge pull request #11 from JocelynDelalande/patch-2

djangorestframework is not a test_require