fix size of retina image

The retina image wasn't being sized correctly.  I just added the two extra lines from the original Chosen css: [here](https://github.com/harvesthq/chosen/blob/master/sass/chosen.scss#L415-L416)

(before and after)
<img src="http://i.imgur.com/psK3vyU.png" width=200px> <img src="http://i.imgur.com/bxjkqMr.png" width=200px>