Inherit host and port from `rails server`

[sikachu]

This makes it so that it will tries to use the defaults given by Rails when user doesn't have listen configuration in their unicorn.rb.

Before this patch:

=> No listen in config/unicorn.rb

$ rails server
=> Booting Unicorn
=> Rails 4.1.5 application starting in development on http://0.0.0.0:3000
=> Run `rails server -h` for more startup options
=> Ctrl-C to shutdown server
I, [2014-09-02T22:06:46.940472 #66762]  INFO -- : listening on addr=0.0.0.0:8080 fd=11

$ rails server -p 3001
=> Booting Unicorn
=> Rails 4.1.5 application starting in development on http://0.0.0.0:3001
=> Run `rails server -h` for more startup options
=> Ctrl-C to shutdown server
I, [2014-09-02T22:06:46.940472 #66762]  INFO -- : listening on addr=0.0.0.0:8080 fd=11

=> With listen 5000 in config/unicorn.rb

$ rails server
=> Booting Unicorn
=> Rails 4.1.5 application starting in development on http://0.0.0.0:3000
=> Run `rails server -h` for more startup options
=> Ctrl-C to shutdown server
I, [2014-09-02T22:06:46.940472 #66762]  INFO -- : listening on addr=0.0.0.0:5000 fd=11

$ rails server -p 3001
=> Booting Unicorn
=> Rails 4.1.5 application starting in development on http://0.0.0.0:3001
=> Run `rails server -h` for more startup options
=> Ctrl-C to shutdown server
I, [2014-09-02T22:06:46.940472 #66762]  INFO -- : listening on addr=0.0.0.0:5000 fd=11

After this patch:

=> No listen in config/unicorn.rb

$ rails server
=> Booting Unicorn
=> Rails 4.1.5 application starting in development on http://0.0.0.0:3000
=> Run `rails server -h` for more startup options
=> Ctrl-C to shutdown server
I, [2014-09-02T22:06:46.940472 #66762]  INFO -- : listening on addr=0.0.0.0:3000 fd=11

$ rails server -p 3001
=> Booting Unicorn
=> Rails 4.1.5 application starting in development on http://0.0.0.0:3001
=> Run `rails server -h` for more startup options
=> Ctrl-C to shutdown server
I, [2014-09-02T22:06:46.940472 #66762]  INFO -- : listening on addr=0.0.0.0:3001 fd=11

=> With listen 5000 in config/unicorn.rb

$ rails server
=> Booting Unicorn
=> Rails 4.1.5 application starting in development on http://0.0.0.0:3000
=> Run `rails server -h` for more startup options
=> Ctrl-C to shutdown server
I, [2014-09-02T22:06:46.940472 #66762]  INFO -- : listening on addr=0.0.0.0:5000 fd=11

$ rails server -p 3001
=> Booting Unicorn
=> Rails 4.1.5 application starting in development on http://0.0.0.0:3001
=> Run `rails server -h` for more startup options
=> Ctrl-C to shutdown server
I, [2014-09-02T22:06:46.940472 #66762]  INFO -- : listening on addr=0.0.0.0:5000 fd=11

Note that it might looks weird for the last case, but that's an expected outcome given user overrides config/unicorn.rb themselves.

Fixes #9

[sikachu]

Would you mind taking another look at this?

[samuelkadolph]

Released as 2.2.0

[sikachu]

Awesome. Thank you!