[fix] Update bad variable reference

lib/winston-mongodb.js

@@ -8,7 +8,7 @@
 
 var util = require('util'),
     mongodb = require('mongodb'),
-    Transport = require('winston').Transport;
+    winston = require('winston');
     
 //
 // ### function MongoDB (options)
@@ -43,7 +43,7 @@ var MongoDB = exports.MongoDB = function (options) {
 //
 // Inherit from `winston.Transport`.
 //
-util.inherits(MongoDB, Transport);
+util.inherits(MongoDB, winston.Transport);
 
 //
 // Define a getter so that `winston.transports.MongoDB`