[AppVeyor] Downgrade npm (3.6.0 has earlier node-gyp w/o cafile)
[AppVeyor] Downgrade npm (3.6.0 has earlier node-gyp w/o cafile)

jdesboeufs/node-gdal,jwoyame/node-gdal,naturalatlas/node-gdal,naturalatlas/node-gdal,nbuchanan/node-gdal,jdesboeufs/node-gdal,naturalatlas/node-gdal,jdesboeufs/node-gdal,jwoyame/node-gdal,jwoyame/node-gdal,jdesboeufs/node-gdal,naturalatlas/node-gdal,nbuchanan/node-gdal,nbuchanan/node-gdal,nbuchanan/node-gdal,jwoyame/node-gdal