Bump Sidekiq to 5.2.5

Stan Hurequested to merge
sh-bump-sidekiq-5.2.5 into master

This reduces duplication caused by gitlab-monitor already shipping Sidekiq 5.2.5. The only changes appear to be minor:

https://github.com/mperham/sidekiq/blob/master/Changes.md

rack-protection changes: https://github.com/sinatra/sinatra/blob/master/CHANGELOG.md

Merge request reports