setup.py requires an unversioned Flask and werkzeug>=0.14

There have been some breaking changes from flask and werkzeug regarding the Request.is_xhr attribute ( see https://github.com/pallets/flask/search?q=is_xhr&type=issues )

While the latest versions are compatible and can be upgraded by users, it would be nice if httpbin updated setup.py to use the current minimal versions - as this can break CI tests if an outdated version of either is picked up.