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
Merge pull request #220 from mbrictson/sidekiq3

Add support for Sidekiq 3.0