Update Travis CI configuration for Windows?

[fletcher]

I added a .travis.yml config file for MMD-6. It seems to work fine (compiles on Debian, passes the tests).

I would like to automatically test building for Windows as well. On my machines, I compile for Windows using Ubuntu and MinGW, though there are other ways to do it.

I welcome input from Travis experts and Windows users about the best approach here.

Thanks!