Set CI to build in Windows and MacOS environment

[rffontenelle]

The current Travis CI implementation (both 'master' and 'py3' branch) is only on a Ubuntu environment. Would be nice to have also Windows and MacOS environment too, to make sure it build and run properly.

Maybe @llimeht want to take a look at it?

[friedelwolff]

Thank you for the suggestion, Rafael. I'm happy if someone wants to look into that and provide a merge request.

I'm probably not personaly going to look into platforms apart from Linux before Linux is in a very good state and we can make a good release on Linux. But I'm definitely not going to block it. Contributions will be very welcome. Virtaal 0.7.1 ran very well on Windows.

MacOS is a different matter. It was not yet at a great level of support, but maybe this is a way to get it started and make it easier for a new contributor if the infrastructure is in place?