fix(auth-plain): only use BSON -after- requiring it

Author: mbroadst

lib/auth/plain.js

@@ -1,13 +1,13 @@
 'use strict';
 
-var BSON = retrieveBSON();
-
 var f = require('util').format,
-  Binary = BSON.Binary,
   retrieveBSON = require('../connection/utils').retrieveBSON,
   Query = require('../connection/commands').Query,
   MongoError = require('../error').MongoError;
 
+var BSON = retrieveBSON(),
+  Binary = BSON.Binary;
+
 var AuthSession = function(db, username, password) {
   this.db = db;
   this.username = username;