Possibly fix "maintainer-shell-script-fails-syntax-check" message

Cause is cryptic, but could be due to the shebang:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=478192

source/debian/postinst

-#!/bin/sh
+#!/bin/bash
 
 set -e
 

source/debian/postrm

-#!/bin/sh
+#!/bin/bash
 
 set -e