pkg-version: Use /bin/bash instead of /bin/sh

`function` is reserved word only available in /bin/bash,
Using `/bin/sh` with `function` keyword fails on Ubuntu.

Change-Id: Ia0fff8f13ace2a0eb1dac50093ff42294d960814
BUG: 1074919
Reviewed-on: http://review.gluster.org/7439
Reviewed-by: Bala FA <barumuga@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>