Sets non-interaction flag for scroll analytics events

Before, ScrollTo events were not being flagged as non-interaction events,
so google was factoring them into bounce rates. This flags ScrollTo events
as non-interactive to fix this.

Info on non-interactive flag:
https://developers.google.com/analytics/devguides/collection/gajs/eventTrackerGuide#non-interaction
Merge pull request #425 from alphagov/non-interaction-flag-for-scroll-tracking

Sets non-interaction flag for scroll analytics events