Introducing: ActiveJob, and support for other queue adapters

• This removes the gem's direct dependency on delayed_job, in favor of an indirect dependency via activejob. As such, other DB-backed queue adapters can theoretically be supported!
• As a safety measure, the gem will now check in production that the queue adapter is :delayed_job, :que, :good_job, or :delayed. If it isn't in that list, the application will fail to boot. (Other adapters are allowed in non-production environments, and this gem now uses the :test adapter for its own tests.)
• The new job class is called Journaled::DeliveryJob. The DelayedJob-friendly equivalent (Journaled::Delivery) is still around for backwards-compatibility with any in-flight jobs. This will be removed in 4.0.
• This also trims down this gem's overall dependencies. Rather than depending on rails, it depends on railties, activejob, and activerecord. (And in its tests, it no longer depends on pg but instead uses an in-memory sqlite database.)