fix(build): enforce strict mode

fixes aurelia/app-ux-showcase#16

Author: JeroenVinke

lib/build/utils.js

@@ -1,3 +1,4 @@
+'use strict';
 const path = require('path');
 const crypto = require('crypto');
 

lib/resolve/index.js

@@ -1,4 +1,4 @@
-let core = require('./lib/core');
+var core = require('./lib/core');
 exports = module.exports = require('./lib/async');
 exports.core = core;
 exports.isCore = function(x) { return core[x]; };