在生产服务器上部署之后，puma会收到错误：
2017-06-06 17:30:33 +0000: Rack app error handling request { GET / }
#<NoMethodError: undefined method `call' for Puma:Module>
/home/deployer/.rvm/gems/ruby-2.2.2/gems/puma-3.9.0/lib/puma/configuration.rb:224:in `call'
/home/deployer/.rvm/gems/ruby-2.2.2/gems/puma-3.9.0/lib/puma/server.rb:602:in `handle_request'
/home/deployer/.rvm/gems/ruby-2.2.2/gems/puma-3.9.0/lib/puma/server.rb:435:in `process_client'
/home/deployer/.rvm/gems/ruby-2.2.2/gems/puma-3.9.0/lib/puma/server.rb:299:in `block in run'
/home/deployer/.rvm/gems/ruby-2.2.2/gems/puma-3.9.0/lib/puma/thread_pool.rb:120:in `call'
/home/deployer/.rvm/gems/ruby-2.2.2/gems/puma-3.9.0/lib/puma/thread_pool.rb:120:in `block in spawn_thread'
Ubuntu 16.04，Ruby 2.2.2，RoR 5.0.3，Puma 3.9。