Upgrade Ruby to 2.0.0-p598

https://www.ruby-lang.org/en/news/2014/11/13/rexml-dos-cve-2014-8090/ offers a new approach to the REXML expansion vulnerability.

Ruby 2.0.0-p598 fixes that.

Changelog: http://svn.ruby-lang.org/repos/ruby/tags/v2_0_0_598/ChangeLog