Fix link to Distributed Ruby (DRb).

The link to DRb is currently broken. A safe and reliable option is to point it to the latest ruby documentation.

![screen shot 2014-02-15 at 22 35 19](https://f.cloud.github.com/assets/246377/2179566/758bcc96-96bb-11e3-8286-fa4d567feef5.png)