update the recommended ruby version.

I changed the message since this file hasn't been updated for more than one year and it sounded like as if we should stick to Ruby 2.0. It appears Rails 4 initially had a problem with Ruby 2.1, but by [March, 2014](http://web.archive.org/web/20140330023208/http://rubyonrails.org/download) (when Rails 4.1 wasn't released yet) the official site started recommending Ruby 2.1.