在ubuntu中使用composer安装laravel v 5.4时出错

hey a get error when install laravel v-5.4

得到错误：

无法下载“http://packagist.org/p/provider-2016%24e217873bdc8408956b33b1eb317ad1182d3a82fde9a746cc5e7def6135696f96.json”文件（HTTP / 1.1 500内部服务器错误）http://packagist.org无法完全加载，包信息从本地缓存加载并且可能已过期安装laravel / laravel（v5.5.0） - 安装laravel / laravel（v5.5.0）：从缓存加载在博客中创建的项目@php -r“file_exists（' . env' ）|| copy（' . env.example'，' . env'）;“使用包信息加载composer存储库更新依赖项（包括require-dev）

无法解码响应：zlib_decode（）：数据错误重试退化模式，请检查 https://getcomposer.org/doc/articles/troubleshooting. md＃degraded-mode以获取更多信息无法将您的要求解析为可安装的软件包集 .

Problem 1
- phpunit/phpunit 6.4.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.4.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.3.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.3.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.2.4 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.2.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.2.2 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.2.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.2.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.1.4 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.1.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.1.2 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.1.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.1.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.9 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.8 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.7 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.6 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.5 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.4 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.2 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.13 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.12 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.11 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.10 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- phpunit/phpunit 6.0.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
- Installation request for phpunit/phpunit ~6.0 -> satisfiable by phpunit/phpunit[6.0.0, 6.0.1, 6.0.10, 6.0.11, 6.0.12, 6.0.13, 6.0.2, 6.0.3, 6.0.4, 6.0.5, 6.0.6, 6.0.7, 6.0.8, 6.0.9, 6.1.0, 6.1.1, 6.1.2, 6.1.3, 6.1.4, 6.2.0, 6.2.1, 6.2.2, 6.2.3, 6.2.4, 6.3.0, 6.3.1, 6.4.0, 6.4.1].

请帮我完成这个问题 . 我试过但没有结果 .