Enqueue customizer script later at wp_enqueue_scripts action

Enqueueing the script at customize_preview_init is too early.
Per http://codex.wordpress.org/Function_Reference/wp_enqueue_script#Notes

> The function should be called using the wp_enqueue_scripts action
> hook if you want to call it on the front-end of the site, like in
> the examples above. To call it on the administration screens, use
> the admin_enqueue_scripts action hook. For the login screen, use the
> login_enqueue_scripts action hook. Calling it outside of an action
> hook can lead to problems, see the ticket #11526 for details.