oneOf example in README does not deserialize  #156

{
  "title": "MultipleObjects",
  "id": "foo",
  "type": "object",
  "oneOf":[
    {"$ref": "#/definitions/ErrorResponse"},
    {"$ref": "#/definitions/VersionGetResponse"}
  ],
  "definitions": {
    "ErrorResponse": {
      "title": "Error Response",
      "id": "Error Response",
      "type": "object",
      "properties": {
        "message": {"type": "string"},
        "status": {"type": "integer"}
      },
      "required": ["message", "status"]
    },
    "VersionGetResponse": {
      "title": "Version Get Response",
      "type": "object",
      "properties": {
        "local": {"type": "boolean"},
        "version": {"type": "string"}
      },
      "required": ["version"]
    }
  }
}

$ ipython
>>> import sys
>>> sys.version
'3.7.1 (default, Oct 22 2018, 11:21:55) \n[GCC 8.2.0]'
>>> import python_jsonschema_objects as pjs
>>> pjs.__version__
'0.3.5'

>>> builder = pjs.ObjectBuilder("oneOf-example.json")
>>> classes = builder.build_classes(named_only=True)

# create two test objects
>>> er = classes.ErrorResponse(message="Danger!", status=99)
>>> vgr = classes.VersionGetResponse(local=False, version="1.2.3")

# round-trip serialize-deserialize into named classes
>>> er2 = classes.ErrorResponse.from_json(er.serialize())
>>> vgr2 = classes.VersionGetResponse.from_json(vgr.serialize())

# round-trip serialize-deserialize into class defined with `oneOf`
>>> er2 = classes.Multipleobjects.from_json(er.serialize())
>>> vgr2 = classes.Multipleobjects.from_json(vgr.serialize())
Traceback (most recent call last):
  File "<ipython-input-10-53386795083b>", line 1, in <module>
    vgr2 = classes.Multipleobjects.from_json(vgr.serialize())
  File "/home/reece/.virtualenvs/default-3.7.1/lib/python3.7/site-packages/python_jsonschema_objects/classbuilder.py", line 116, in from_json
    obj.validate()
  File "/home/reece/.virtualenvs/default-3.7.1/lib/python3.7/site-packages/python_jsonschema_objects/classbuilder.py", line 272, in validate
    .format(missing, self.__class__.__name__))
ValidationError: '['message', 'status']' are required attributes for ErrorResponse