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
The `line-height: normal` of the input makes sure that the text is centered within the input, no matter what height it is. The 5px padding was never necessary actually, regardless of border-box or content-box. So nothing ends up changed with what I did here.