Fix Converse RAI - $msg is not defined (#144)

plugins/converse-rai.js

@@ -75,13 +75,13 @@
     {
         console.debug("sendMarker", to_jid, id, type);
 
-        const stanza = $msg({
+        const stanza = converse.env.$msg({
           'from': _converse.connection.jid,
           'id': Math.random().toString(36).substr(2,9),
           'to': to_jid,
           'type': 'chat'
         }).c(type, {
-          'xmlns': Strophe.NS.MARKERS,
+          'xmlns': converse.env.Strophe.NS.MARKERS,
           'id': id
         });