Fix strict standard notice

I am seeing the following error when using your plugin: 

>PHP Strict Standards:  Declaration of ACF_Walker_Taxonomy_Field_Checklist::start_el() should be compatible with Walker::start_el(&$output, $object, $depth = 0, $args = Array, $current_object_id = 0) in .../taxonomy-field.php on line 513

Just need to update the args used by start_el methods to be compatable with the `Walker::start_el`