如何使用现有的Symfony 2项目安装symfony cmf？

I would like to install Symfony CMF into an existing Symfony2 project.

I am using this documentation.

After running php composer.phar update, I get the following error:

[RuntimeException]

  Could not read http://packagist.org/packages-dev-n.json, either you or the remote host is probably offline.

  The "http://packagist.org/packages-dev-n.json" file could not be downloaded

update [--prefer-source] [--dry-run] [--dev] [--no-scripts] [packages1] ... [packagesN]

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What am I doing wrong?