Retrieves the schema registered under the given subject with
        the given version id. Returns the schema as a `dict`.

def get_subject_version(self, subject, version_id):
        """
        Retrieves the schema registered under the given subject with
        the given version id. Returns the schema as a `dict`.
        """
        res = requests.get(self._url('/subjects/{}/versions/{}', subject, version_id))
        raise_if_failed(res)
        return json.loads(res.json()['schema'])