The "https://repo.packagist.org/p/laravel/laravel.json" file could not be downloaded: failed to open

composer 安装laravel5.5 报错：

[Composer\Downloader\TransportException] 
The "https://repo.packagist.org/p/laravel/laravel.json" file could not be downloaded: failed to open stream: Connection timed out

或者报错：

[Composer\Downloader\TransportException]
The "http://packagist.org/packages.json" file could not be downloaded: failed to open stream: HTTP request failed!

解决办法，更改composer的源为阿里的地址即可。

有关composer使用请参照阿里镜像源文档：https://developer.aliyun.com/composer?spm=a2c6h.13651102.0.0.3e221b11PUXlFo

更换composer源：

#更改composer镜像源为国内地址
composer config -g repo.packagist composer https://mirrors.aliyun.com/composer/