Remove sidekiq/job_retry dependency

[stefanste]

I have made an oversight in the recent fix PR #42 and not required this. Without it when the fallback max retry attempts value gets accessed, Sidekiq::JobRetry::DEFAULT_MAX_RETRY_ATTEMPTS is not defined.

Included a spec which fails when the sidekiq/job_retry module is not required.

Thanks for reviewing and merging the previous PR promptly, apologies for missing this!

[kaisen-san]

No worries. Thanks for putting a fix together! Seems like some tests failed, though. Could you look into it?

[stefanste]

Thanks @kaisen-san I've pushed a fix that should sort it.

It was failing due to Sidekiq v4 defining that constant in a different place to 5+: https://github.com/sidekiq/sidekiq/blob/fe43e1cbcd0498a37a69581ace523bc36630cd3f/lib/sidekiq/middleware/server/retry_jobs.rb#L67

We could do something like wrap the require in a a begin/rescue, and then require it from the different location to support Sidekiq 4. But maybe better not to reach into Sidekiq's internals and just define the constant here to match? Let me know what you think.

[kaisen-san]

Yeah, that makes sense to me!