Fix for delayed_job loading

Hi bugsnag-team,

`Delayed::Job` does not have `Delayed::Plugins::Plugin`. The `Plugin` class inherited in official [clear_locks](https://github.com/collectiveidea/delayed_job/blob/v3.0.0/lib/delayed/plugins/clear_locks.rb) plugin is `Delayed::Plugin`.

Could you fix this issue?

https://github.com/collectiveidea/delayed_job/blob/v3.0.0/lib/delayed/plugin.rb
https://github.com/collectiveidea/delayed_job/blob/v4.0.0/lib/delayed/plugin.rb