Update the logo for Retina devices

The logo was all messed up on retina devices. This commit fixes that! (see attached screenshots)

![screenshot 2014-02-19 17 46 32](https://f.cloud.github.com/assets/855037/2208967/b2593eb6-9985-11e3-9312-054bea6426c5.png)
![screenshot 2014-02-19 17 46 09](https://f.cloud.github.com/assets/855037/2208968/b25aa35a-9985-11e3-8379-d603ab75ed3d.png)