Auto-generating PHPCR proxies in debug mode.

Without this setting fixture loading and the store were not working.

```
PHP Warning:  require(/var/www/sylius/app/cache/dev/doctrine/PHPCRProxies/__CG__DoctrineODMPHPCRDocumentGeneric.php): failed to open stream: No such file or directory in /var/www/sylius/vendor/doctrine/common/lib/Doctrine/Common/Proxy/AbstractProxyFactory.php on line 207
```