Clear overdue flag if duedate or SLA changes and the ticket is no longer overdue.
Use GMT to compare due dates.
Merge pull request #518 from protich/issue/overdue_revisited

Clear overdue flag on ticket update

If the due date or SLA changes and  pushes the actual deadline into the
future and the ticket is currently overdue, then drop the overdue flag.

Reviewed-By: Jared Hancock <jared@osticket.com>