Fix wercker link

README.md

 * [Shippable](https://app.shippable.com/) - A docker based continious integration platform for open source and private projects.
 * [Sismo](http://sismo.sensiolabs.org/) - A continuous testing server library.
 * [Travis CI](https://travis-ci.org/) - A continuous integration platform.
-* [Wercker](http://wercker.com/) - A continuous integration platform
+* [Wercker](http://www.wercker.com/) - A continuous integration platform
 
 ## Documentation
 *Libraries for generating project documentation.*