Scrollbar fix

Style adjustments to codemirror:
![screen shot 2014-10-01 at 5 12 30 pm](https://cloud.githubusercontent.com/assets/108094/4483360/c8955b98-49b0-11e4-8626-e60b149a84d1.png)

Fix for https://github.com/mapbox/mapbox-studio/issues/767