Depend on published vector-tile-js version

[lukasmartinelli]

Can we depend on a stable https://github.com/mapbox/vector-tile-js version?

https://github.com/mapbox/vt2geojson/blob/f5e6f1f5afedb5c7f9b6ef225e25207cf8a4b5fc/package.json#L22

[jfirebaugh]

We'd either need to get https://github.com/mapbox/vector-tile-js/pull/33 merged and do a major version release of vector-tile-js, or port that code to vt2geojson itself. My preference is the former; the ring classification code is already duplicated in several projects and it would be good not to add another.