Add option for nonInteraction set to true

This matches the previous analytics track event function.

From @36degrees:

The sixth element in the trackEvent array means means that opt_noninteraction is true, which means that the 'event hit will not be used in bounce-rate calculation'.

as per https://developers.google.com/analytics/devguides/collection/analyticsjs/events#non_interaction_events

javascripts/stageprompt.js

@@ -63,7 +63,9 @@
 
   GOVUK.performance.sendGoogleAnalyticsEvent = function (category, event, label) {
     if (global.ga && typeof global.ga === 'function') {
-      global.ga('send', 'event', category, event, label)
+      global.ga('send', 'event', category, event, label, {
+        nonInteraction: true
+      })
     } else {
       global._gaq.push(['_trackEvent', category, event, label, undefined, true])
     }