Fix layout to follow bc CSS

This is related with #12 and try to address [my comment](https://github.com/swcarpentry/lesson-template/pull/12#issuecomment-62623131).

![swc](https://cloud.githubusercontent.com/assets/1506457/5001129/002a97aa-69da-11e4-8f3a-5802b9982541.jpg)