Update CmfBlockExtension to prevent overriding sonata_block_render

An exception has been thrown during the rendering of a template ("Notice: Undefined offset: 1 in /vendor/symfony-cmf/block-bundle/Symfony/Cmf/Bundle/BlockBundle/Templating/Helper/CmfBlockHelper.php line 64")

The template code that triggered this error was similar to:
```jinja
{{ sonata_block_render({ 'name': '/blocks/Test/case' }, { 'template': 'ExampleBundle:Test:Block/block_simple.html.twig' }) }}
```

The issue appears when the `new Sonata\BlockBundle\SonataBlockBundle(),` is before the `new Symfony\Cmf\Bundle\BlockBundle\CmfBlockBundle(),` in a Symfony2 Standard Edition app/AppKernel.php registerBundles() method.