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
well, the behavior previously was

```
padding-top 5px + content height 15px + padding-bottom 5px (+border 0)  ==> border height 25px
```

The new sets the height to 25px on the border-box, which is the same height for the element. But it changes the display *inside* the element, by giving a height of 25px to the content instead of having 15px for the content and a small padding around it (unless another rule sets some padding for inputs on the page of course)