`bin/dev`したらブラウザで自動でlocalhost:3000開く設定

config.ru

# This file is used by Rack-based servers to start the application.

require_relative 'config/environment'

run Rails.application
Rails.application.load_server

Launchy.open('http://localhost:3000') if Rails.env.development? # 追加

Gemfile

group :development do
  ## (略)
  gem 'launchy' # 追加
  ## (略)
end