Fix invalid highlighting regexp literal

Please review this patch

### before

![coffee-regexp-before](https://f.cloud.github.com/assets/554281/1754749/06f616bc-6658-11e3-9117-693cc8e33b7b.png)


### after

![coffee-regexp-highlight](https://f.cloud.github.com/assets/554281/1754751/0a7f3566-6658-11e3-8f58-0d5d711d4a84.png)