Fix dead link in README.md

The [old link](http://andywaite.com/2011/5/18/rspec-disable-nulldb), returns a 404 error.  The [new link](https://web.archive.org/web/20120419204019/http://andywaite.com/2011/5/18/rspec-disable-nulldb) points to a cached copy of the original article.