fix(ci): Use pytest-django dev for django dev (#792)

Author: untitaker

tox.ini

@@ -82,7 +82,8 @@ deps =
     {py2.7,py3.7,py3.8}-django-{1.11,2.2,3.0,3.1,dev}: psycopg2-binary
 
     django-{1.6,1.7,1.8}: pytest-django<3.0
-    django-{1.9,1.10,1.11,2.0,2.1,2.2,3.0,3.1,dev}: pytest-django>=3.0
+    django-{1.9,1.10,1.11,2.0,2.1,2.2,3.0,3.1}: pytest-django>=3.0
+    django-dev: git+https://github.com/pytest-dev/pytest-django#egg=pytest-django
 
     django-1.6: Django>=1.6,<1.7
     django-1.7: Django>=1.7,<1.8