Add Travis build status badge to README

Author: justinfagnani

README.md

@@ -1,6 +1,8 @@
 webcomponents.js
 ================
 
+[![Build Status](https://travis-ci.org/webcomponents/webcomponentsjs.svg?branch=master)](https://travis-ci.org/webcomponents/webcomponentsjs)
+
 [![Join the chat at https://gitter.im/webcomponents/webcomponentsjs](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/webcomponents/webcomponentsjs?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 A suite of polyfills supporting the [Web Components](http://webcomponents.org) specs: