Migrate to new VS 2017 RC AppVeyor image to fix test logger issue

[daviwil]

Following up from #351:

In its WMF 5 image, AppVeyor currently doesn't have a test logger compatible with the latest .NET CLI tools so we can't get our dotnet test output to show up in AppVeyor's test list yet. We are still able to use the traditional test run approach to get by for now, but we should move to the new image and get rid of the old approach one Feodor gives us access to the beta image.