update tox to django 2.0 beta1 (#5507)

Author: auvipy

tox.ini

@@ -24,7 +24,7 @@ setenv =
 deps =
         django110: Django>=1.10,<1.11
         django111: Django>=1.11,<2.0
-        django20: Django>=2.0a1,<2.1
+        django20: Django>=2.0b1,<2.1
         djangomaster: https://github.com/django/django/archive/master.tar.gz
         -rrequirements/requirements-testing.txt
         -rrequirements/requirements-optionals.txt