Fix log subscriber

[shahidkhaliq]

Using both rails_semantic_logger and activejob-uniqueness together in Rails 6.0.x results in duplicated logs as reported in #73.

The Issue

1. Rails 6.0.x has ActiveJob::Logging::LogSubscriber.attach_to :active_job.
2. activejob-uniqueness has ActiveJob::LogSubscriber.attach_to :active_job_uniqueness.
3. rails_semantic_logger expects to see one of these but replaces both of these with it's own logger here. And thus duplicating logging.

The Solution

This PR ensures that we are creating and attaching a unique log subscriber ActiveJob::Uniqueness::LogSubscriber and not creating the ActiveJob::LogSubscriber class in older Rails versions.

[shahidkhaliq]

@sharshenov could you take a look, please?