fix highlighting ruby classes

The background-color made selections invisible on ruby clases

before:
![screen shot 2014-11-10 at 10 51 24 am](https://cloud.githubusercontent.com/assets/1312687/4979787/d831b67e-68f9-11e4-91f1-4f7d7874fae3.png)

after:
![screen shot 2014-11-05 at 2 10 26 pm](https://cloud.githubusercontent.com/assets/1312687/4925044/e5f678b0-6527-11e4-82c5-f602299347ef.png)