Modify GitHub pages domain

Now this page is served as `rack.github.io`.
* https://github.com/blog/1452-new-github-pages-domain-github-io

If this PR is acceptable, could you update repository description?
![2013-06-11 20 32 19](https://f.cloud.github.com/assets/290782/636654/04f019fa-d28b-11e2-9c6a-49bf697132de.png)