Replace dead railsapi.com with lively RubyDocs.org

The railsapi.com site has been dead for many months and [the project is moribund](https://github.com/voloko/railsapi.com). It makes sense to replace a dead link with a worthwhile replacement and that's exactly what RubyDocs.org is.

The only issue I have with RubyDoc.org for now is that it's not open source but I pinged @manume (the maintainer) on Twitter to see if he could open source what runs [RubyDocs.org](http://rubydocs.org) and [he said he would soon](https://twitter.com/manume/status/464054555429195776).