parse-sass: don't hardcode bash path

It's /bin/bash on some systems; just use /bin/sh instead.

https://bugzilla.gnome.org/show_bug.cgi?id=749593

gtk/theme/Adwaita/parse-sass.sh

@@ -1,3 +1,3 @@
-#!/usr/bin/bash
+#!/bin/sh
 
 bundle exec sass --update --sourcemap=none .