Add support for Sidekiq 3.0

In Sidekiq 3.0, the mechanism for registering error handlers has changed. Instead of installing a middleware class, a Proc needs to be registered.

This commit adds the 3.0 mechanism while still supporting < 3.0. It also includes a test case for the 3.0 behavior.

See also: https://github.com/mperham/sidekiq/blob/master/3.0-Upgrade.md