Fixed Cerberus canary unit test

Apparently it's no longer possible for Cerberus to validate its own schemas.

tests/test_api/test_cerberus.py

@@ -15,6 +15,8 @@ class CerberusCanaryTest(unittest.TestCase):
         groups_schema = {'name': {'type': 'string', 'required': True}}
 
         # On error, validate_schema() raises ValidationError
+        if hasattr(validator, 'validate_schema'):
+            # It was removed in Cerberus 1.0 (or thereabouts)
             validator.validate_schema(groups_schema)
 
         # On error, validate() returns False