Fix universal analytics check

We used to be checking for the existence of Universal Analytics with `window.ga` but really we should have been using a more durable option which is checking for it via Google Analytics object, which defends against both people renaming their stuff as well as rogue plugins defining window.ga in other ways. The new way is to check the existence of `window[window.GoogleAnalyticsObject]` and send via that instead.

Props to @bschwartz for the help!