update to use latest topojson command line binary

Since the [2.0.0 release](https://github.com/topojson/topojson/releases), `topojson` has been replaced by `geo2topo` as the command line tool.  The Makefile should be updated to use the latest binary.  

Additionally, there is now a [topo2geo](https://github.com/topojson/topojson-client/blob/master/README.md#topo2geo) that can probably replace the `topo2geojson.py` python script.