Upgrade to Sidekiq 3.0

In the latest sidekiq, the redis pool is passed through the middleware. Also the ```Sidekiq::Queue``` class is defined in ```sidekiq/api```. All specs pass and it's working for us in production.

See https://github.com/mperham/sidekiq/blob/master/3.0-Upgrade.md for the official 3.0 release notes.