Composer is operating significantly slower than normal because you do not have the PHP curl extension enabled

运行composer update的时候，composer 报警：

Composer is operating significantly slower than normal because you do not have the PHP curl extension enabled.

这虽然不是致命错误，但会严重拖慢 composer 的速度，因此必须解决，方法很简单，安装php-curl扩展就可以了：

sudo apt install php-curl