[Rake] Correctly deal with pagination

There we're a couple of issues with the current implementation:

- It only tried 2 pages, wouldn't try any more
- It incorrectly duplicated page 2 of the contributors count. Since the
  underlying API didn't support pagination. It was doing `?page=2` to
  which returned the same page. Thus duplicating and missing users.

These changes move to GitHub 3 API to get contributors which supports
pagination.
Update contributors