Use SVG badge instead of PNG badge

| before | after |
| ---- | ---- |
| [![Build Status](https://travis-ci.org/cujojs/when.png?branch=master)](https://travis-ci.org/cujojs/when) | [![Build Status](https://travis-ci.org/cujojs/when.svg?branch=master)](https://travis-ci.org/cujojs/when) |

SVG badges look beautiful on retina displays.