travis jobs: enable Windows

[joshmoore]

Follow on to #132. Run Windows in Travis.

[joshmoore]

All platforms are running (periodically with tables failures) and a real issue with the Windows test arose:

>           return open(string, self._mode)
E           FileNotFoundError: [Errno 2] No such file or directory: 'C:UserstravisAppDataLocalTemppytest-of-unknownpytest-0popen-gw3testLoad0test'

[joshmoore]

Green!

[manics]

One comment, but otherwise looks good

[joshmoore]

Few commits pushed, @manics.

[manics]

Might need script: skip too
https://docs.travis-ci.com/user/build-stages/#build-stages-and-deployments

[joshmoore]

That's us back green again, @manics. Not sure why both skips are needed.

[manics]

I've no idea either. skip_cleanup means don't cleanup the earlier build step, but since this is using build stages which are isolated from each other the results of the other stages aren't available so there shouldn't be anything that could be cleaned

[joshmoore]

¯\_(ツ)_/¯ Note: this may have further issues when it comes time to deploy a tag, but I'll deal with that during release.