change display time of average time in staff leaderboard when there's no grading

change 99:99:99 to --:--:--

Before:
![screen shot 2014-12-30 at 11 40 22 am](https://cloud.githubusercontent.com/assets/4983239/5575487/12577e92-9019-11e4-9d92-e792f8fa7090.png)

Now:
![screen shot 2014-12-30 at 11 38 17 am](https://cloud.githubusercontent.com/assets/4983239/5575488/198c31b2-9019-11e4-8790-7fc97f100346.png)