[ci skip] Update Ruby versions.

https://www.ruby-lang.org/en/news/2014/11/13/ruby-2-1-5-is-released/
https://www.ruby-lang.org/en/news/2014/11/13/ruby-2-0-0-p598-is-released/
https://www.ruby-lang.org/en/news/2014/11/13/ruby-1-9-3-p551-is-released/