Merge pull request #1568 from mongodb/NODE-1173

fix(import): adds missing import to lib/authenticate.js

Author: daprahamian

lib/authenticate.js

@@ -1,6 +1,7 @@
 var shallowClone = require('./utils').shallowClone
   , handleCallback = require('./utils').handleCallback
-  , MongoError = require('mongodb-core').MongoError;
+  , MongoError = require('mongodb-core').MongoError
+  , f = require('util').format;
 
 var authenticate = function(self, username, password, options, callback) {
   // Did the user destroy the topology