解决办法：先用 composer diagnose 命令检测
然后 composer self-update 更新composer版本
最后执行 composer update 或者 composer install
composer 切换阿里云镜像 用起来还快
composer config -g repo.packagist composer https://mirrors.aliyun.com/composer/
来源：https://my.oschina.net/u/3911977/blog/3106312