Fix path to composer.lock file

`$container->getParameter('kernel.root_dir')` returns `root/app/cache/cms/../..`
while `$kernel->getRootDir()` returns `root/app`

So doing `dirname` on that just removes the last `/..` part.
`$composerPath` is then `root/app/cache/composer.lock` instead of `root/composer.lock`

This throws an exception:

    [2014-05-27 13:59:55] request.CRITICAL: Uncaught PHP Exception Kunstmaan\AdminBundle\Helper\VersionCheck\Exception\ParseException: "No composer.lock file found in your project root" at root/vendor/kunstmaan/admin-bundle/Kunstmaan/AdminBundle/Helper/VersionCheck/VersionChecker.php line 150 {"exception":"[object] (Kunstmaan\\AdminBundle\\Helper\\VersionCheck\\Exception\\ParseException: No composer.lock file found in your project root at root/vendor/kunstmaan/admin-bundle/Kunstmaan/AdminBundle/Helper/VersionCheck/VersionChecker.php:150)"} {"user":{"username":"admin","roles":{"0":"ROLE_SUPER_ADMIN","1":"ROLE_PERMISSIONMANAGER","2":"ROLE_ADMIN","4":"ROLE_USER"},"is_account_non_expired":true,"is_account_non_locked":true,"is_credentials_non_expired":true,"is_enabled":true},"file":"root/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/EventListener/ExceptionListener.php","line":104,"class":"Symfony\\Component\\HttpKernel\\EventListener\\ExceptionListener","function":"logException"}

This seems to occur on the `login_check` route for me and on the settings bundle versions page.