Update mod_sic according to xep-0279 version 0.2

The specification for xep-0279 has changed: http://xmpp.org/extensions/xep-0279.html, so I modified mod_sic to reflect the changes.

If accepted, an update of https://github.com/esl/ejabberd_tests/blob/master/tests/sic_SUITE.erl is ready to follow as a pull request also!
You can safely replace `list_to_binary(integer_to_list(Port))` with `interger_to_binary(Port)`. For older Erlang/OTPs we have a hack defined in [ejabberd.hrl:71-86](https://github.com/esl/MongooseIM/blob/master/apps/ejabberd/include/ejabberd.hrl#L71-L86)