RetryJobs#retry_attempts_from is presently a private method

Pasted from error in sidekiq:

```private method `retry_attempts_from' called for #<Sidekiq::Middleware::Server::RetryJobs:0x0000000761ccc0>```

As of [this](https://github.com/mperham/sidekiq/commit/a42e4960c14d9bdca22747f9c51a8c0617762215#diff-e0641eb66d77ea31a898bffe0a375076) commit, retry_attempts_from is a private method. I've (roughly) duplicated it's behavior here.