Update Google Analytics snippet to Universal Analytics

This is the update of the GA snipped to Google Universal Analytics
Please see https://github.com/h5bp/html5-boilerplate/commit/48d49e96d6db282eb9686d31ebbc5cbbbdd4d966 and h5bp/html5-boilerplate#1347 and h5bp/html5-boilerplate#1369 for more information on this topic.

Head tip @mathiasbynens.