Use updated ref_count logic from rdf-rdfa.
Override rdf-rdfa preprocessing for RDF/XML semantics in managing prefixes.

This addresses issue #31.
Update dependencies to RDF.rb and rdf-rdfxml gems.
Version 1.1.2.
Release 1.1.2.

* Use updated ref_count logic from rdf-rdfa.
* Override rdf-rdfa preprocessing for RDF/XML semantics in managing prefixes.