update JS file improvements

wpbu() sets e.src = "//browser-update.org/update.min.js";

Improvements:

1. it should be HTTPS, not protocol-relative, since it could be intercepted and will work even if the WP site is HTTP-only
2. it should use wp_enqueue_script() because that’s the right way

If using wp_enqueue_script(), I could override it to load it from https://www.jsdelivr.com/package/npm/browser-update, for example