Add background to pre to improve legibility

## Before

![before](http://f.cl.ly/items/1u1W0M373r2I1j2a3r0f/Screen%20Shot%202014-07-08%20at%204.39.13%20PM.png)

## After

![after](http://f.cl.ly/items/2Y1P0J452z1N260y2V1i/Screen%20Shot%202014-07-08%20at%204.41.36%20PM.png)

Could also use a darker font color with a lighter background.