[RAILS 6] Deprecation warning when specifying server

Code:

guard 'rails', server: 'puma', port: 3001 do
  watch('Gemfile.lock')
  watch(%r{^(config|lib)/.*})
end

Generates this warning on run:

DEPRECATION WARNING: Passing the Rack server name as a regular argument is deprecated and will be removed in the next Rails version. Please, use the -u option instead. (called from <main> at bin/rails:4)
=> Booting Puma
=> Rails 6.0.0 application starting in development 
=> Run `rails server --help` for more startup options
Puma starting in single mode...
* Version 3.12.1 (ruby 2.5.6-p201), codename: Llamas in Pajamas
* Min threads: 5, max threads: 5
* Environment: development
* Listening on tcp://localhost:3001
Use Ctrl-C to stop
14:06:55 - INFO - Rails started, pid 28504

Guard version 2.15.0

ranmocy / guard-rails

[RAILS 6] Deprecation warning when specifying server #51