build-rebar3: Append postPatch to original hook

The postPatch attribute is already in the function argument but is
unused elsewhere in the code, so setting postPatch with buildRebar3 is
going to end up in a no-op.

This now allows to use postPatch within buildRebar3 by simply appending
to the existing postPatch hook that removes the rebar and rebar3
escripts.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @ericbmerritt