Use ES6 in lib files.

lib/common/util.js

@@ -1,6 +1,6 @@
 'use strict';
 
-var _ = require('lodash');
+const _ = require('lodash');
 
 /*----------------------------------------------------------------------------*/
 
@@ -13,7 +13,7 @@ var _ = require('lodash');
  * @returns {Object} Returns the new hash object.
  */
 function Hash(properties) {
-  return _.transform(properties, function(result, value, key) {
+  return _.transform(properties, (result, value, key) => {
     result[key] = (_.isPlainObject(value) && !(value instanceof Hash))
       ? new Hash(value)
       : value;
@@ -35,6 +35,6 @@ function pitch(error) {
 }
 
 module.exports = {
-  'Hash': Hash,
-  'pitch': pitch
+  Hash,
+  pitch
 };