Postiats version 0.3.0

The attached patch updates the Postiats nixpkgs expression to the
latest released 0.3.0 version.

From c89bfabd847ebd62c8dd77cb7385ea4effa929be Mon Sep 17 00:00:00 2001
From: Karn Kallio <kkallio@skami.org>
Date: Sat, 21 Jan 2017 13:28:23 -0400
Subject: [PATCH] ats2 : update Postiats to the latest released 0.3.0 version.