readme: add travis build badge

This adds [![Build Status](https://travis-ci.org/visionmedia/node-querystring.png?branch=master)](https://travis-ci.org/visionmedia/node-querystring) to the readme.

Stylish!

[![Posh cat](http://i.imgur.com/zX9u568.gif)](http://gifsound.com/?gif=i.imgur.com/zX9u568.gif&v=6t28COxEp2k&s=26)