fix(build): fixed minor issue in systemjs builder, and added SystemJs cache buster

public/app/boot.js

@@ -1,6 +1,15 @@
 (function bootGrafana() {
   'use strict';
 
+  var systemLocate = System.locate;
+  System.locate = function(load) {
+    var System = this;
+    return Promise.resolve(systemLocate.call(this, load)).then(function(address) {
+      return address + System.cacheBust;
+    });
+  };
+  System.cacheBust = '?bust=' + Date.now();
+
   System.import('app/app').then(function(app) {
     app.default.init();
   }).catch(function(err) {