refactored to use PrismicBundle

this PR requires that https://github.com/lsmith77/PrismicBundle is added to packagist.org. I guess it would make sense to also move that Bundle to the prismicio organization on github beforehand. The code is a slightly refactored version of the code I removed from this repo.

Once this is done the composer.lock file will also need to be updated.

Note in order to clean up the use of the Request object in the Prismic Helper, I had to increase the minimal Symfony2 version to 2.4+. It would be possible to keep it at 2.3 if we really really wanted to ..