any_of='use-first' not working

[kurtrwall]

Describe the bug
Btw, great library, thanks for making it!

I'm trying to use this with draft 6 jsonschema (Microsoft's AdaptiveCard schema, and I'm still getting:

NotImplementedError: anyOf is not supported as bare property (workarounds available by setting any_of flag)

Example Schema and code

import python_jsonschema_objects as pjs
import requests
schema = requests.get('https://adaptivecards.io/schemas/adaptive-card.json').json()
ns = builder.build_classes(any_of='use-first')
...snip
NotImplementedError: anyOf is not supported as bare property (workarounds available by setting any_of flag)

Expected behavior
The schema was parsed and the classes were generated.