fix(client): add ES5 shim

Karma client now uses core-js ES5 shim in order to support IE8.
Usage of dom-serializer had previously broken IE8 support.

Closes #1529

client/main.js

@@ -1,6 +1,7 @@
 /* global io */
 /* eslint-disable no-new */
 
+require('core-js/modules/es5')
 var Karma = require('./karma')
 var StatusUpdater = require('./updater')
 var util = require('./util')