use bash instead of sh because -a is a bash specific option. fixes #1620

launch.sh

-#!/bin/sh
+#!/bin/bash
 cd `dirname $0`/src
 exec -a gajim python -OOt gajim.py $@

scripts/gajim

-#!/bin/sh
+#!/bin/bash
 ##      gajim
 ##
 ## Contributors for this file: