Upgrade net-ssh, sidekiq and dokku version

[michiels]

This PR fixes a bunch of issues by updating the dependencies for net-ssh, sidekiq, and Dokku.

• net-ssh update fixes an issue with RSA key API changed
• sidekiq updated to recent version, also to make use of autoloading in Rails 5+
• Dokku version updated because previous installation script was broken because of a wrong Dokku Apt repository

While doing this, a few things broke in the app so I had to fix them too:

• In the ServerInstallJob make sure the channels stays open. Otherwise we cannot capture all SSH output. This must be related to the net-ssh gem update.
• Fix and clean several configurations items in the CircleCI config

[michiels]

Wow, that was a lot of "going down the rabbit hole" to fix up some gems to their recent forms to get master unbroken. Luckily everything works now. Going to merge this into continue cleaning up the other PRs in this repository and start on new features.