Use GeoTrellis' unioned method

GeoTrellis now has a union method; the import
for CascadedPolygonUnion in GeoTrellis broke
this set of code since the imports conflicted.

See: https://github.com/geotrellis/geotrellis/pull/906

This commit fixes that and uses the new unioned
method in GeoTrellis.
Merge pull request #411 from WorldBank-Transport/feature/fix-geotrellis-union

Use GeoTrellis' unioned method