throw an exception when invalid geom passed to Sieve

[mappingvermont]

Our current implementation of Sieve will return null for any invalid geometry. This PR updates Sieve to throw an error if it's passed an invalid geom, which can be optionally be handled in an exception when others block.

This also changes the area_threshold type to anyelement, allowing one to pass both float and integer values as area_threshold.

There's also some housekeeping in .travis.yml - bumping the postgres, postgis, ubuntu and node versions.

[nyurik]

@mappingvermont we are actively working on https://github.com/openmaptiles/postgis-vt-util , updating it with parallel support, up to date PG/PostGIS versions, better testing, etc. You might want to submit PRs there too, but note that your code doesn't seem to be parallel friendly - do you think it would be possible to make it parallel executable to allow for speed ups?

[mappingvermont]

👋 hey @nyurik !

Thanks for tagging me in this- I wasn't aware of the openmaptiles repo. I don't have much experience writing parallel-friendly pl/pgsql, but will try and update it if I have some free time.

Feel free to PR as well if that's easier. Thanks!