Rails 5.1 is merged into Discourse.
One notable change which was a source of pain in development for me is that Sidekiq supports live reloading with Rails 5 but dependencies in each file has to be required properly. Otherwise, the job will be stucked in the Sidekiq queue. I’ve fixed most that I can find but if you run into any other job, feel free to let me know.
https://github.com/discourse/discourse/commit/23b787e0a6a695ff8b70d095eb6d0ce603f28f15