update jwt_decode_handler to match pyjwt decode signature

change the method invocation to use new kwargs instead of current positional args to match pyjwt

pyjwt: 
* [https://github.com/progrium/pyjwt/blob/master/jwt/__init__.py#L257](https://github.com/progrium/pyjwt/blob/master/jwt/__init__.py#L257)
* [https://github.com/progrium/pyjwt/blob/master/jwt/__init__.py#L305]
(https://github.com/progrium/pyjwt/blob/master/jwt/__init__.py#L305)

tests passed using `./runtests.py`