Appveyor: build on Windows Server 2012 R2 and Windows Server 2016
Appveyor provides build images with Windows Server 2012 R2 (the default) and Windows Server 2016. These images are called:
- Visual Studio 2015
- Visual Studio 2017
Our builds work on these images: https://ci.appveyor.com/project/teor2345/tor/builds/20046114
So we should probably build on both of them.