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

1. It makes the leaderboard more fair.
2. Tested properly, it works.
this part ?

submission = asm.submissions.where(std_course_id: user_course_id).first

On Tue, Sep 9, 2014 at 11:20 PM, Raymoond Tang <notifications@github.com>
wrote:

> In app/models/user_achievement.rb:
>
> > @@ -4,4 +4,26 @@ class UserAchievement < ActiveRecord::Base
> >    belongs_to :user_course
> >    belongs_to :achievement
> >
> > +  def update_date
>
> can you tell why your code is "inefficient" ?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/Coursemology/coursemology.org/pull/204/files#r17307129>
> .
>