Removed .to_json on RDF::Value and other classes

Defining .to_rdf is too broad, and messes with use in JSON-LD. I changed it to just use .to_rdf_json, which I'm sure will be fine, and does not have undesirable side-effects.

If you add me as an owner on RubyGems, I can release it if you like. In the mean time, I'm going to comment it out of the linkeddata gem, as it causes the distiller to crash when trying to serialize JSON-LD.