Fix bug #8725R: replace ST_simplify by ST_SnapToGrid to avoid collapsed geometries by postgis

This pull replaces ST_Simplify by ST_SnapToGrid to simplify the geometries in postgis provider.

It avoids a bug in postgis driver that collapse geometries (and them dissapear in the map) when the simplifcation factor is too large.

See discussion:
http://lists.osgeo.org/pipermail/postgis-devel/2012-December/023152.html
http://trac.osgeo.org/postgis/ticket/1987

SnapToGrid may return more vertices, but the result is safer:
http://blog.cartodb.com/post/20163722809/speeding-up-tiles-rendering