Small fix on configuration

If you have a route configured like that (a var with a $) :
```
route_index:
    pattern:  /route
    defaults:
        _controller: sylius.controller.route:indexAction
        _sylius:
            paginate: $limit
```

The method get of the Configuration class will not return the default value
```php
$this->get('paginate', 10); // return null if paginate is not defined instead of 10
```