Repair 'fix' for analytics scroll tracker non-interaction flag

The analytics scroll tracker failed to set the non-interaction flag, and so
was having an influence on the bounce rate of tracked pages. A previous
commit tried to correct that, but missed one of the google analytics
event paramaters.
Merge pull request #429 from alphagov/fix-non-interaction-flag-for-scroll-tracking

Repair 'fix' for analytics scroll tracker non-interaction flag