Update Rake task to only run on Sundays

Since the Rake task to populate the database will now be run
everyday by Heroku, create a version of the job that will only
run on Sundays to avoid using unneeded time on Heroku.
Update stubbing of GitHub API in tests

Prior to this commit, one of the calls to the GitHub API
(organization_member?) was not being stubbed in the tests which
caused failures due to API rate limits.

Now the return value of this method is stubbed to always be "true"