Configure tests for Django 2.1

src/auditlog_tests/router.py

@@ -33,6 +33,6 @@ class PostgresRouter(object):
         Make sure the postgres app only appears in the 'postgres db'
         database.
         """
-        if model_name.startswith('postgres'):
+        if model_name and model_name.startswith('postgres'):
             return db == 'postgres'
         return None

tox.ini

@@ -4,6 +4,7 @@ envlist =
     {py27,py34,py35,py36}-django-110
     {py27,py34,py35,py36}-django-111
     {py34,py35,py36}-django-20
+    {py35,py36}-django-21
 
 [testenv]
 setenv =
@@ -13,7 +14,8 @@ deps =
     django-18: Django>=1.8,<1.9
     django-110: Django>=1.10,<1.11
     django-111: Django>=1.11,<2.0
-    django-20: Django>=2.0
+    django-20: Django>=2.0,<2.1
+    django-21: Django>=2.1,<2.2
     -r{toxinidir}/requirements-test.txt
 basepython =
     py36: python3.6