Update customSelect.jquery.js

Fixes keyboard interaction with the select box in Firefox - unlike other browsers, FF doesn't fire "change" event until the select element loses focus, meaning the span display wasn't updating properly. By adding a "keyup" event in addition to the change event, this limitation is bypassed and the plugin works for keyboard users in FF.
Merge pull request #12 from mpettitt/patch-1

Fix for Firefox "change" event with keyboard use