I updated one of my apps to Rails 5 and upgraded the Ruby version to 2.3.1 as well. The app already used Puma prior to the Rails 5 upgrade as well and was deployed on a Digital Ocean droplet.
When I start
=> Booting Puma
=> Rails 5.0.0 application starting in development on http://localhost:3000
=> Run `rails server -h` for more startup options
 Puma starting in cluster mode...
 * Version 3.4.0 (ruby 2.3.1-p112), codename: Owl Bowl Brawl
 * Min threads: 5, max threads: 5
 * Environment: development
 * Process workers: 2
 * Preloading application
 * Listening on tcp://localhost:3000
 Use Ctrl-C to stop
 - Worker 1 (pid: 14684) booted, phase: 0
 - Worker 0 (pid: 14683) booted, phase: 0
Resolved this issue, and confirmed by @marvindanig who was experiencing the same issue, that the 'tmp' folder needed to be cleared. There is a rake task in rails to do so...