[skip travis] AppVeyor: explicitly install 'node-gyp'
[skip travis] AppVeyor: explicitly install 'node-gyp'

mapbox/node-pre-gyp,mojodna/node-pre-gyp,mojodna/node-pre-gyp,mapbox/node-pre-gyp,mapbox/node-pre-gyp,mojodna/node-pre-gyp,mapbox/node-pre-gyp,mojodna/node-pre-gyp,mapbox/node-pre-gyp,mojodna/node-pre-gyp