Add support for Sidekiq 3.x

It seems like the API that delayed_paperclip uses internally doesn't change from Sidekiq 2.x to 3.x. The only thing that changed is that you now have to explicitly `require sidekiq/api` in the test files to get access to `Sidekiq::Queue`. Changing `require sidekiq` to `require sidekiq/api` seems to work with both Sidekiq versions. At least it works for me locally with and without an update to Sidekiq 3.x.