use st_force2d instead of deprecated st_force_2d (#4803)
use version specific postgis force2d function (#4803)

also adds a PROCESSING "FORCE2D=no" to avoid using force2d altogether (#4024)
Allows floating point NoData/NULLVALUEs for floating point images.
Merge PR #4803
fix travis build
Merge branch 'branch-6-4'