This contains only minor changes that tighten up some dependencies:

* https://github.com/mperham/sidekiq/blob/master/Changes.md
* https://github.com/mperham/sidekiq/compare/v5.2.7...v5.2.9

gitlab-exporter is using v5.2.9 already, so this change avoids duplicate
versions being shipped in Omnuibus.

Note we avoid v5.2.8 since this unnecessarily restricts Rack.