Retry on failure (including 4xx failure) to get the git repo info.

Summary:
Hopefully this will help us track down why we're sometimes getting
   ['message']
instead of real data from our github list-repos API call!

Test Plan:
Ran
   ./update_phabricator_repositories.py -n
and saw it list all the github repos we have, as well as indicate it
was fetching all 4 pages of repo-info.

Reviewers: chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D3952