Fix sonata bootstraping Kernel requirement

We have the following error on Jenkins:

```
PHP Warning:  include_once(/data/www/<integ>/app/Admin/AdminKernel.php): failed to open stream: No such file or directory in /data/www/<integ>/app/console on line 48
```

I think this is because `sonata_bootstrap()` try to require a non-lower named directory in `app` directory.

This should fix the problem.