Make all of Chosen use border-box

Just found out about https://github.com/harvesthq/chosen/pull/2009

* First, we're using compass for box-sizing (FF28 and before requires `-moz`), so wanted to make that content-box addition use compass.
* Then I wondered why we're defining `border-box` in so many places. We should just override all of Chosen to be on the safe side, so I did that instead.
* I only had to update 2 height values with this change.
* This would close https://github.com/harvesthq/chosen/pull/2037

@stof @tjschuck
but we don't have the 5px padding anymore now, which is not the same behavior