Delayed Job Support

[orangewolf]

Hi,
Wanted to see how you felt about adding Delayed::Job support back in to the health monitor. I've added basic queue size support and thought we'd get a PR open before we go any further in developing it.

Totally open to thoughts and feedback.

[coveralls]

Coverage increased (+0.06%) to 99.435% when pulling 76d3321 on notch8:delayed_job into 27abc80 on lbeder:master.

[lbeder]

Thanks for the contribution, @orangewolf! Could you also update the README.md accordingly? In the meantime, let me sync the master branch with the latest version of rubocop (which makes the tests to fail for this PR).

[lbeder]

Could you also fix the following rubocop offenses?

Offenses:
health-monitor-rails.gemspec:34:3: C: Gemspec/OrderedDependencies: Dependencies should be sorted in an alphabetical order within their section of the gemspec. Dependency delayed_job_active_record should appear before sidekiq.
  s.add_development_dependency 'delayed_job_active_record', '>= 4.1'
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
spec/lib/health_monitor/providers/delayed_job_spec.rb:17:1: C: Layout/EmptyLinesAroundBlockBody: Extra empty line detected at block body beginning.

[orangewolf]

Updated to add readme and fix rubocop warnings

[lbeder]

Can you fix the failing tests? Your PR, unfortunately, still doesn't pass the tests :(

[lbeder]

Well done! 👏👏

I'll merge and release this soon as 8.1.0.