New CircleCI badge.

Hi! CircleCI engineer here. We released a new, prettier, retina-friendly badge, and noticed you were still on the old one.

What do you think?
![screen shot 2014-12-10 at 4 29 04 pm](https://cloud.githubusercontent.com/assets/490421/5387108/addfca94-8089-11e4-8e77-680bd3263665.png)