Replace the obsolete method warn.

http://docs.python.org/3/library/logging.html

  Note: There is an obsolete method warn which is functionally identical to
  warning. As warn is deprecated, please do not use it - use warning instead.

src/common/connection_handlers_events.py

@@ -1207,7 +1207,7 @@ class GcInvitationReceivedEvent(nec.NetworkIncomingEvent):
             try:
                 self.room_jid = helpers.parse_jid(invite_tag.getAttr('jid'))
             except helpers.InvalidFormat:
-                log.warn('Invalid JID: %s, ignoring it' % invite_tag.getAttr(
+                log.warning('Invalid JID: %s, ignoring it' % invite_tag.getAttr(
                     'jid'))
                 return
             self.jid_from = self.msg_obj.fjid