Remove responsive styles for 640-1024 screen sizes.

### Before

![coderdojo - san francisco-2](https://f.cloud.github.com/assets/623/1812187/a0137dce-6e70-11e3-8d2a-b871281e39f2.jpg)

### After

![coderdojo - san francisco-1](https://f.cloud.github.com/assets/623/1812190/a9cfb382-6e70-11e3-9df1-fad9952bd491.jpg)

/cc @mikefowler