Add a note about ruby 1.9 requirements

modules/exploits/multi/http/rails_xml_yaml_code_exec.rb

@@ -23,6 +23,11 @@ class Metasploit3 < Msf::Exploit::Remote
 				any ruby code remotely in the context of the application.
 
 				This module has been tested across multiple versions of RoR 3.x and RoR 2.x
+
+				The technique used by this module requires the target to be running a fairly version
+				of Ruby 1.9 (since 2011 or so). Applications using Ruby 1.8 may still be
+				exploitable using the init_with() method, but this has not been demonstrated.
+
 			},
 			'Author'         =>
 				[