Setup signal to trap SIGINT and gracefully stop server

[shayonj]

This way, when you send SIGINT to a server, when running via
rails s, a graceful shutdown/stop will be performed.

Before this, not trapping SIGINT would mean that either Rails::Server#start
or ::Rack::Server#start would be trapping SIGINT and exiting the process.

Fix for: #1362

References:

• ::Rack::Server#start's trapping ofSIGINT.

Replication of current/fixed behavior:

rails s
=> Booting Puma
=> Rails 5.1.2 application starting in development on http://localhost:3000
=> Run `rails server -h` for more startup options
Puma starting in single mode...
* Version 3.9.1 (ruby 2.3.3-p222), codename: Private Caller
* Min threads: 5, max threads: 5
* Environment: development
* Listening on tcp://0.0.0.0:3000
Use Ctrl-C to stop
^C- Gracefully stopping, waiting for requests to finish
=== puma shutdown: 2017-07-26 19:53:48 -0700 ===
- Goodbye!
Exiting

[stereobooster]

code lower in this file

if Puma.jruby?
  Signal.trap("INT") do

move those two together

[shayonj]

fixed