升级 php composer 版本

在执行 composer update 时，报错

You made a reference to a non-existent script @php artisan package:discover

网上说是 composer 的版本太低，查看了一下当前安装的版本

composer -V Composer version 1.2.1 2016-09-12 11:27:19 

解决方法，升级 composer 版本

composer selfupdate Updating to version 1.6.3 (stable channel). Downloading: 100% Use composer self-update --rollback to return to version 1.2.1