Use dash instead of bash for postinst

debian/postinst

@@ -1,4 +1,4 @@
-#!/bin/bash -e
+#!/bin/sh -e
 set -e
 
 # Tag to allow some debhelper commands to inject relevant code
@@ -16,7 +16,7 @@ if [ "${1}" = "configure" ]; then
     fi
 fi
 
-if [[ "$1" = "configure" || "$1" = "abort-upgrade" || "$1" = "abort-remove" ]]; then
+if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ] || [ "$1" = "abort-remove" ]; then
     # Theme package alternatives
     while read theme priority; do
         update-alternatives --install \