Updated ruby version to 2.1.5

It fixes a vulnerability in XML processing. See:

https://www.ruby-lang.org/en/news/2014/11/13/rexml-dos-cve-2014-8090/

A few other bugs have also been fixed, see:

https://www.ruby-lang.org/en/news/2014/11/13/ruby-2-1-5-is-released/