Update supported ruby version

Ruby 1.9.2 seems to be no longer supported.
Because the security patch has not been applied to 1.9.2:
* http://www.ruby-lang.org/en/news/2013/06/27/hostname-check-bypassing-vulnerability-in-openssl-client-cve-2013-4073/
Merge pull request #2920 from tricknotes/update-supported-ruby-version

Update supported ruby version