Require six >=1.4.1 for six.moves.urllib

``six.moves.urllib`` was [added in version 1.4.0](https://bitbucket.org/gutworth/six/src/dea5c233c6f404bf3468195ae9ef80497d08258f/CHANGES?at=default#cl-111) and then fixed in 1.4.1, so this adds that requirement explicitly.

This should fix #112.