Use SVG badge in README

Travis now provides a SVG replacement for the PNG status badge, which is much easier to read on retina display. For example, this is the difference from my perspective:

![screen shot 2014-05-08 at 8 35 05 pm](https://cloud.githubusercontent.com/assets/688886/2923570/86b8cc86-d71a-11e3-850c-d2dcc0347071.png)