Restore the abstract on Token model when the app isn't declared

The fact that we don't import Token from authentication doesn't invalidate
the need for the model to be abstract whenever the authtoken isn't listed
in the INSTALLED_APPS.

rest_framework/authtoken/models.py

@@ -22,6 +22,14 @@ class Token(models.Model):
                                 on_delete=models.CASCADE)
     created = models.DateTimeField(auto_now_add=True)
 
+    class Meta:
+        # Work around for a bug in Django:
+        # https://code.djangoproject.com/ticket/19422
+        #
+        # Also see corresponding ticket:
+        # https://github.com/tomchristie/django-rest-framework/issues/705
+        abstract = 'rest_framework.authtoken' not in settings.INSTALLED_APPS
+
     def save(self, *args, **kwargs):
         if not self.key:
             self.key = self.generate_key()