Use six.assertRaisesRegex shim in YubiKeyTest

Prevents a DprecationWarning due to older Django releases calling
TestCase.assertRaisesRegexp() with callable=None.

Also presents a DeprecationWarning on Python 3.4, due to (indirectly)
calling TestCase.assertRaisesRegex**p**().

tests/tests.py

@@ -1096,7 +1096,8 @@ def test_setup(self, verify_token):
         self.assertContains(response, 'YubiKey')
 
         # Without ValidationService it won't work
-        with self.assertRaisesMessage(KeyError, "No ValidationService found with name 'default'"):
+        with six.assertRaisesRegex(self, KeyError,
+                                   "No ValidationService found with name 'default'"):
                 self.client.post(reverse('two_factor:setup'),
                                  data={'setup_view-current_step': 'method',
                                        'method-method': 'yubikey'})