为什么composer update将symfony更新到2.5，但它是';symfony/symfony&qu

也许我对Composer的工作方式完全错误，但考虑到以下composer.json (Symfony的一部分，有一些额外的要求没有在这里显示):

"require": {
    "php": ">=5.3.3",
    "symfony/symfony": "~2.4",
    "doctrine/orm": "dev-master",
    "doctrine/dbal": "dev-master",
    "doctrine/doctrine-bundle": "dev-master",
    "twig/extensions": "~1.0",
    "symfony/assetic-bundle": "~2.3",
    "symfony/swiftmailer-bundle": "~2.3",
    "symfony/monolog-bundle": "~2.4",
    "sensio/distribution-bundle": "~2.3",
    "sensio/framework-extra-bundle": "~3.0",
    "sensio/generator-bundle": "~2.3",
    "incenteev/composer-parameter-handler": "~2.0",
    // my requirements here
}

当我运行composer update时，我得到symfony/symfony 2.5而不是~2.4(即AFAIK 2.4.1, 2.4.2等)。