refactor: use correct Engine.IO protocol revision

The 3rd revision is included in Socket.IO v1/v2, whereas the 4th is
included in Socket.IO v3.

Note: this query parameter is not currently checked on the server-side

Reference: https://github.com/socketio/engine.io-protocol

src/internal/sio_client_impl.cpp

@@ -222,7 +222,7 @@ namespace sio
             } else {
                 ss<<uo.get_host();
             }
-            ss<<":"<<uo.get_port()<<"/socket.io/?EIO=4&transport=websocket";
+            ss<<":"<<uo.get_port()<<"/socket.io/?EIO=3&transport=websocket";
             if(m_sid.size()>0){
                 ss<<"&sid="<<m_sid;
             }