Merge branch 'cs-upgrade-unicorn' into 'master'

Upgrade unicorn and unicorn-worker-killer.

Upgrade unicorn from 4.9.0 to 5.1.0 and unicorn-worker-killer from 0.4.2 to 0.4.4. This includes Rack 2.0 support which is necessary for Rails 5.

Changelog for unicorn: http://unicorn.bogomips.org/NEWS.html

Changelog for unicorn-worker-killer: https://github.com/kzk/unicorn-worker-killer/blob/bdec225f028d1bda01b5888578c479b0c705f1a8/ChangeLog

See merge request !6463