fix(angulartics-ga.js): Check for ga before _gaq

Right now, angulartics-ga.js checks for `window._gaq` before `window.ga` to decide what form of event tracking to use. Since, with the [new tracker script](https://developers.google.com/analytics/devguides/collection/upgrade/reference/gajs-analyticsjs#snippet), both variables are declared on the window object, only the `_gaq` form of event tracking is used. That causes event tracking to fail.

This makes event tracking work with the new tracker script by moving the check for `window.ga` to before the check for `window._gaq`.