http: use util._deprecationWarning for deprecation warning

lib/http.js

@@ -1590,8 +1590,8 @@ exports._connectionListener = connectionListener;
 
 function Client(port, host) {
   // TODO http.Client can be removed in v0.9. Until then leave this message.
-  console.trace('http.Client is a legacy interface and will be removed in ' +
-                'the near future. Do not use it.');
+  util._deprecationWarning('http.Client', 'http.Client is a legacy interface' +
+                ' and will be removed in the near future. Do not use it.');
   host = host || 'localhost';
   port = port || 80;
   this.host = host;