assert VoID dataset "hasPart" to evidence if set

Asserts the "Dataset hasPart Evidence" triple only if the
void_dataset_uri option is set when translating to RDF.

refs #66

lib/bel/translator/plugins/rdf/writer.rb

@@ -39,7 +39,10 @@ def <<(evidence)
         statements.each do |statement|
           @writer.write_statement(statement)
         end
-        @writer.write_statement(RDF::Statement.new(@void_dataset_uri, RDF::DC.hasPart, evidence_uri))
+
+        if @void_dataset_uri
+          @writer.write_statement(RDF::Statement.new(@void_dataset_uri, RDF::DC.hasPart, evidence_uri))
+        end
       end
 
       def done