Fixes GDK issue where repos would not be imported properly

Seed-Fu runs this entire fixture in a transaction, so the `after_commit`
hook won't run until after the fixture is loaded. That is too late
since the Sidekiq::Testing block has already exited. Force clearing
the `after_commit` queue to ensure the job is run now.

See: gitlab-org/gitlab-development-kit#58

See merge request !1513