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

用 composer 方式安装框架 yii2.0 前，需要安装 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 来解决。