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

launch.sh

@@ -1,3 +1,3 @@
-#!/bin/sh
+#!/bin/bash
 cd `dirname $0`/src
 exec -a gajim python -OOt gajim.py $@

scripts/gajim

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 ##      gajim
 ##
 ## Contributors for this file: