Use svg instead of png to get better image quality

README.md

-# reveal.js [![Build Status](https://travis-ci.org/hakimel/reveal.js.png?branch=master)](https://travis-ci.org/hakimel/reveal.js)
+# reveal.js [![Build Status](https://travis-ci.org/hakimel/reveal.js.svg?branch=master)](https://travis-ci.org/hakimel/reveal.js)
 
 A framework for easily creating beautiful presentations using HTML. [Check out the live demo](http://lab.hakim.se/reveal-js/).