fix empty element on depdrop with select2

Hi!
If I use depdrop with select2 and on select2 enable multiselect, on dep select add empty element.
https://github.com/kartik-v/dependent-dropdown/blob/master/js/dependent-dropdown.js#L22 

http://i63.fastpic.ru/big/2014/0926/69/617a051f91b1a51f7c6adef6d91efb69.jpg