Upgrade Sidekiq from 4.1.4 to 4.2.1, remove dependency on Sinatra

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

Sinatra is no longer required and sidekiq uses a vanilla Rack app for
its Web UI now.