Update for Weekly Leaderboard API endpoint

The current weekly and monthly leaderboard methods make requests that no longer appear supported. When using those methods the summary, lifetime, average, etc. values in the response are all zero.

Browsing the API documentation, I found [this](https://wiki.fitbit.com/display/API/API-Get-Friends-Leaderboard) endpoint that returns the last 7 days data. This pull request updates the `weekly_leaderboard` to call that endpoint instead.