Allow for default number of retry attempts to be set for RetryJobs middleware.

* Adds `max_retries` option to RetryJobs middleware. Default is still 25. This
  way you can still retry jobs, but not have it spread out over such a long
  period.

Modify `Sidekiq::Middleware::Chain#add` semantics

* If middleware class already exists in the chain, remove the
  existing class and add it with possibly new arguments.