Instantiate DelayedJob::Worker only once.

Prior to delayed_job v4.0.2, the previous code worked. After v4.0.3, we
started getting the following failure:

1) DelayedJob integration when it's installed when a method is delayed
and an exception occurs notifies Honeybadger
   Failure/Error: expect(Honeybadger).to receive(:notify_or_ignore)
     (Honeybadger).notify_or_ignore(any args)
         expected: 1 time with any arguments
         received: 2 times with any arguments
   # ./spec/honeybadger/integrations/delayed_job_spec.rb:58:in `block
   # (5 levels) in <top (required)>'