Menu Customizer: explicitly enqueue `accordion.js`, which will be required in WordPress 4.1 with the addition of the new JS API for sections and panels (which removes the core need for `accordion.js` in the Customizer).

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@1013140 b8457f37-d9ea-0310-8a92-e5e31aec5664