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!