Fix or ditch appveyor

[bsipocz]

The appveyor CI got broke down quite badly in the past month.

I would be inclined to pull the plug, as we already planned, but first we need to check how many active projects still use it. It maybe just a false impression that many have already moved on to travis or azure to do windows testing.

[astrofrog]

I would vote for ditch - we could keep the script and print out an error message recommending alternatives?

[pllim]

👍 to ditch. Adding a os: windows in .travis.yml isn't that hard.