update achievement gain date to submission date(not grading date)

1. It makes the leaderboard more fair.
2. Tested properly, it works.
well, updated_at is a column that activerecord will auto update upon save, so actually even you assign a value here, upon calling uach.save, updated_at will be override to the current time. 

What I will do is to add a new column: obtained_at

and name update_date to something like assign_obtained_date