Try to fix intermittent SSL error in RebaseTest.has_rebased_pr()

Attempt to fix intermittent SSLerror like http://ci.openmicroscopy.org/view/Failing/job/SCC-merge/378/console. Alternatively, we could extend `GithubRepository.get_pulls()` to allow a `filter` argument which would make it inherit the `retry_on_error` decoration.