portability: use /bin/sh instead of /bin/bash

There is nothing requiring bash specific features in this script, so
just use /bin/sh which is portable amongst all Unices.

https://bugzilla.gnome.org/show_bug.cgi?id=679847

src/gnome-shell-extension-prefs.in

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 if [ -n "$GI_TYPELIB_PATH" ]; then
     export GI_TYPELIB_PATH=@pkglibdir@