update circleci badge

Hi! CircleCI engineer here. We released a new, prettier, retina-friendly badge, and noticed you were still on the old one :)

![screenshot 2014-12-10 16 51 54](https://cloud.githubusercontent.com/assets/1475986/5387299/f2ded7d6-808c-11e4-8d73-60b879bf4473.png)