io-console.gemspec: certification

* ext/io/console/io-console.gemspec: add a certification.
  http://www.benjaminfleischer.com/2013/11/08/how-to-sign-your-rubygem-cert/

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43662 b2dd03c8-39d4-4d8f-98ff-823fe69b080e