Twig extension broken

[flack]

It seems that the latest pull request broke the twig extension:

https://travis-ci.org/symfony-cmf/cmf-sandbox/jobs/3162736/#L1463

The fun thing is that the unit tests still pass, but that's because the twig extension unittests use their own implementation of RdfFactory. I'm not exactly sure why that was done (intuitively, I would say it should be a mock object instead), and I don't have a symfony testbed for this, so @dbu, could you take a look at it if you have some time?

[dbu]

uh oh. yes i will try to fix this.

[flack]

This is now fixed, with #37 as a followup for better detection of these conditions in the unittests