The "https://packagist.org/packages.json" file could not be downloaded

通过 Composer 安装 yii2.0 前，需安装 Composer Asset 插件，该过程如果报错

The "https://packagist.org/packages.json" file could not be downloaded，原因是

系统请求 https://packagist.org/ 受阻，可通过 设置系统网络代理 解决。