Upgrade Sidekiq to 4.2.1, remove dependency on Sinatra

This updates Sidekiq to 4.2.1, which adds full support for Rails 5 by removing a dependency on Sinatra which was one of the remaining Rails 5 blockers.

Major things to check: Sidekiq still works, obviously. Also that the Web UI/Admin dashboard works and doesn't lose any functionality (based on my testing it works fine).

Working toward #14286.

Changelog: https://github.com/mperham/sidekiq/blob/921e939f995fbb5238975d4121d728b95be99ab5/Changes.md#421

See merge request !6349