add iceTransports: relay, supported in chrome 39+

libs/strophe/strophe.jingle.js

     connection: null,
     sessions: {},
     jid2session: {},
-    ice_config: {iceServers: []},
+    ice_config: {iceServers: [], iceTransports: 'relay'},
     pc_constraints: {},
     media_constraints: {
         mandatory: {