下载安装包
curl -sS https://getcomposer.org/installer | php
把 composer 把复制到 /usr/bin/composer
mv composer.phar /usr/bin/composer
composer 运行，ok。
caomposer报错，以及解决方法：
Failed to decode response: zlib_decode(): data error
Retrying with degraded mode, check https://getcomposer.org/doc/articles/troubleshooting.md#degraded-mode for more info
解决方法是：
composer -vvv update