composer create-project laravel/laravel --prefer-dist blog
prefer:提升，提拔
dist:距离
blog 创建的项目文件夹名称
laravel设置下载安装的源：
composer config -g repo.packagist composer https://packagist.laravel-china.org
指定安装的laravel版本：(安装5.4的版本laravel)
composer create-project laravel/laravel laravel54 "5.4.*"