Do not require raven.dsn when a client_id is specified

Right now, I have the following configuration. And as you can see, the raven.dsn
is useless here, are the client_id service is not buildt by the extension
but by myself.

```yml
monolog:
    handlers:
        raven:
            type: raven
            # useless, because we set-up a custom client, but we have to...
            dsn:  %raven.dsn%
            client_id: sensiolabs.toolkit.monolog.raven_client
```

So with with patch, we can use:

```yml
monolog:
    handlers:
        raven:
            type: raven
            client_id: sensiolabs.toolkit.monolog.raven_client
```

BTW, I don't know the target branch of this PR.