Update NPM on AppVeyor (old versions don't understand "^")

The bundled version of NPM that comes with the 0.8 node binary is old and doesn't understand `^0.5.0`-style version ranges, which causes this error: https://ci.appveyor.com/project/Mapbox/node-pre-gyp/build/1.0.23/job/xc2r3kpshgpfhbe9#L207

We ran into this with [node-gdal](https://github.com/naturalatlas/node-gdal) and this fixed it.