io-console.gemspec: drop 1.9

* ext/io/console/io-console.gemspec: drop 1.9 support since
  rb_cloexec_open() is mandatory now.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42473 b2dd03c8-39d4-4d8f-98ff-823fe69b080e