composer self-update 失败

全局更换镜像：
composer config -g repo.packagist composer https://packagist.phpcomposer.com

composer config -g repo.packagist composer https://packagist.laravel-china.org

或修改项目中的composer.json文件：

"repositories": {
    "packagist": {
          "type": "composer",
          "url": "https://packagist.phpcomposer.com"
      }
  }