Use admin_url() instead of home_url() to get the AJAX url

This PR changes the home_url() call to admin_url().

By using *home_url()* to get an URL to wp-admin you assume too much of the directory structure of the site. For instance, we usually keep WordPress in a separate directory from our index.php, leading the wp-admin url to be *http://site.com/wordpress/wp-admin* while home_url() returns *http://site.com/*.