Bring postgis simplification factor back to 1/20 of a pixel

This was the value in 2.2.x and was accidentally changed as part
of a completely unrelated commit, see:
http://github.com/mapnik/mapnik/issues/1639#issuecomment-53388951
Merge pull request #2393 from strk/2.3.x-pgis-simplify-20th

Bring postgis simplification factor back to 1/20 of a pixel