Don't load Sidekiq/Resque jobs unless the correspending backend is defined.

Fixes an issue where requiring the gem without sidekiq crashes:

```
lib/apn/jobs/sidekiq_notification_job.rb:6:in `<class:SidekiqNotificationJob>': uninitialized constant APN::Jobs::SidekiqNotificationJob::Sidekiq (NameError)
```