Uuse ordereddict pypi package for 2.6

`flask_restful.utils` includes the `OrderedDict` implementation by [Raymond Hettinger](https://github.com/flask-restful/flask-restful/blob/83eb935d30f7b029b6798e18b01841e3d076cab5/flask_restful/utils/ordereddict.py#L1), which is already available on PyPI, so it seems pointless to include it here.