fix(serializer): remove use of const

Author: mbroadst

lib/bson/parser/serializer.js

@@ -6,7 +6,7 @@ var writeIEEE754 = require('../float_parser').writeIEEE754,
   Map = require('../map'),
   Binary = require('../binary').Binary;
 
-const normalizedFunctionString = require('./utils').normalizedFunctionString;
+var normalizedFunctionString = require('./utils').normalizedFunctionString;
 
 // try {
 //   var _Buffer = Uint8Array;