Drop precision of geojson files

[bhousel]

The build script does some automatic processing of the geojson files already to:

• enforce right-hand winding order
• prettify the output

We should also drop the double precision coordinates within the files to make them smaller. 5 decimal places equals about 1 meter resolution at the equator, and is good enough for the purposes of this index.

Most of the files are already pretty small, but this could help shrink germany.geojson and belgium.geojson a lot more.