Make DRF master test run with stable Django

tox.ini

@@ -2,7 +2,7 @@
 envlist =
     py27-django111-drf37,
     py{34,35,36}-django{111,20}-drf37,
-    py36-drfmaster,
+    py36-django20-drfmaster,
     lint, docs
 
 [travis:env]
@@ -17,9 +17,8 @@ deps =
 
     drf37: djangorestframework>=3.7.7,<3.8
 
-    # test with the latest build of Django and django-rest-framework to get early warning of compatibility issues
+    # test with the latest build of django-rest-framework to get early warning of compatibility issues
     drfmaster: https://github.com/encode/django-rest-framework/archive/master.tar.gz
-    drfmaster: https://github.com/django/django/archive/master.tar.gz
 
     # other dependencies
     -rrequirements/validation.txt
@@ -28,9 +27,6 @@ deps =
 commands =
     pytest --cov --cov-config .coveragerc --cov-append --cov-report="" {posargs}
 
-[testenv:py36-drfmaster]
-pip_pre = True
-
 [testenv:lint]
 skip_install = true
 deps =