don't allow to install the split Security packages
don't allow to install the split Security packages

Currently, you would be able to install the Security component fromm
Symfony 2.3 together with one of the split packages from a higher
Symfony vesion like this:

```json
{
    "require": {
        "symfony/symfony": "2.3.*",
        "symfony/security-core": "~2.7"
    }
}
```

However, you will end up with classes being present twice.

This must be reverted after merging up in the `2.7` branch.

symfony/Security