Replace dead railsapi.com with lively RubyDocs.org

The railsapi.com site has been dead for many months and the project moribund:
- https://github.com/voloko/railsapi.com

The only issue I have with RubyDoc.org for now is that it's not open source
but the maintainer is considering making it open source relatively soon:
- https://twitter.com/manume/status/464054555429195776