Require Ruby >= 1.9.3

Last patch version of Ruby 1.9.2 (p320) was released in April 2012, almost a year ago.
Since than 4 patch versions for 1.9.3 were released: : [fix1][1], [fix2][2], [fix3][3]
1.9.2 doesn't contain fixes for them so I think it's not supported

  [1]: http://www.ruby-lang.org/en/news/2012/11/09/ruby19-hashdos-cve-2012-5371/
  [2]: http://www.ruby-lang.org/en/news/2013/02/22/json-dos-cve-2013-0269/
  [3]: http://www.ruby-lang.org/en/news/2013/02/22/rexml-dos-2013-02-22/