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
@stof Any other thoughts on the 5px padding? I'm pretty sure it's not necessary anymore with this change, unless you can give me an example of where we still need it and I'll add it back in.