https://github.com/payjp/payjp-example/tree/master/sinatra が ruby 2.3.0p0 で動かなかった
% ruby -v (git)-[master]
ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin15]
% PUBLIC_KEY=pk_test_0383a1b8f91e8a6e3ea0e2a9 SECRET_KEY=sk_test_c62fade9d045b54cd76d7036 ruby app.rb (git)-[master]
== Sinatra (v1.4.8) has taken the stage on 4567 for development with backup from Puma
Puma starting in single mode...
* Version 3.6.0 (ruby 2.3.0-p0), codename: Sleepy Sunday Serenity
* Min threads: 0, max threads: 16
* Environment: development
* Listening on tcp://localhost:4567
/Users/mori/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puma-3.6.0/lib/puma/single.rb:16:in `stop': undefined method `stop' for nil:NilClass (NoMethodError)
from /Users/mori/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puma-3.6.0/lib/puma/launcher.rb:146:in `stop'
from /Users/mori/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/sinatra-1.4.8/lib/sinatra/base.rb:1424:in `quit!'
from /Users/mori/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/sinatra-1.4.8/lib/sinatra/base.rb:1449:in `ensure in run!'