irb> p Nokogiri::VERSION_INFO
{"warnings"=>[], "ruby"=>{"engine"=>"mri", "version"=>"1.8.7", "platform"=>"i686-linux"},
"libxml"=>{"loaded"=>"2.7.6", "binding"=>"extension", "compiled"=>"2.7.6"}, "nokogiri"=>"1.4.3.1"}

libxml2 のバージョンが低すぎるのだと思う