Make event tracking work with universal analytics

Google Analytics can be included using either the class `ga` code or
the newer universal `analytics` code. Both should work.

https://developers.google.com/analytics/devguides/collection/gajs/eventT
rackerGuide
https://developers.google.com/analytics/devguides/collection/analyticsjs
/events

* Update the track method to push events to both forms of the tracking
code.
Merge pull request #55 from alphagov/universal-analytics-tracking

Make event tracking work with universal analytics