fixed incorrect negation

Calling `this.selected_item_focus_handle.remove()` only makes sense if `this.selected_item_focus_handle` exists in the first place. The `if` statement on line 774 had this logic backwards.

This error:
![screen shot 2014-03-26 at 3 05 04 pm](https://cloud.githubusercontent.com/assets/194735/2530016/f7c76a24-b521-11e3-90d0-ee35897890be.PNG)
would appear when applying Chosen to a disabled `select` element.

Simply removing the `!` fixed it right up.