Switch from findwinsdk to msvs-detect

[dra27]

This extracts part of #17 (which I still want to come back to). findwinsdk is an early version of what became msvs-detect which now lives at https://github.com/metastack/msvs-tools.

The benefits of the switch are threefold:

• It's slightly faster, inasmuch as it detects the x86 and x64 compiler in a single call
• It's maintained, so it detects more recent Visual Studio installations
• It fixes AppVeyor (or at least it did on the run on my fork...)

[dra27]

I intentionally haven't updated the AppVeyor matrix to include recent compilers, pending #78