Inherit host and port from `rails server`

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