git://code.creativecommons.org/license.rdf.git has stopped responding.

So use a cloned copy.
Add upstream CC 4.0 licenses.

The CC RDF git repository is not responding, so these are converted
from the embedded RDFa.
Hack in dc:isReplacedBy pointers from CC 3.x licenses to 4.0.

(should really use the rdflib turtle serializer here, instead of constructing it as a string manually).
Hack in support for CC 4.0 licenses.
JSON-LD literals changed from @literal to @value, allow both for now.
Actually add the CC 4.0 data, and make sure "make clean" keeps them around.