`autenticate` signature doesn't match ModelBackend one

[okainov]

authenticate method from ModelBackend has request as mandatory first argument. In KrbBackend it doesn't have request at all

[02strich]

This seems to be a change in Django since the last update to this code (as I see the parameter in 2.2 but not 1.7 Django). Happy to take a pull request as I don't have a current Django setup to validate the change.