我从使用Symfony 2的Git项目克隆了新项目。然后我尝试更新Composer：
$ composer update
Loading composer repositories with package information
The "https://packagist.org/packages.json" file could not be downloaded: failed to open stream: Connection timed out
https://packagist.org could not be fully loaded, package information was loaded from the local cache and may be out of date
Updating dependencies (including require-dev)
Generating autoload files
[ErrorException]
file_put_contents(/var/www/html/alipay_web/vendor/composer/autoload_namespa ces.php): failed to open stream: Permission denied
所以有人可以告诉我该怎么办？