Appveyor build fails

See https://ci.appveyor.com/project/snoyberg/stack, which fails because http://www.stackage.org/stack/windows-i386 now redirects to a .tar.gz file, but before it redirect to a zip file.

Since Windows users typically have built-in tools to extract from zip, but nothing that handles tar.gz by default, I would have thought sticking with .zip is a better choice.