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.
Merge pull request #54 from amorphid/master

Fix dead link in README.md