GraphicsMagick

This started as an attempt to debug https://github.com/mapbox/tilelive-vector/issues/15 (didn't succeed other than determining that the file size is way off), but I ended up switching the image comparison tests from ImageMagick to GraphicsMagick following the pattern in https://github.com/mapbox/tilestream-pro/pull/3235/files. Next step would be to add support for --update for automatically updating fixtures.

/cc @willwhite