Add RDF versions to Format.

Author: gkellogg

lib/rdf/format.rb

@@ -47,6 +47,10 @@ module RDF
   class Format
     extend ::Enumerable
 
+    ##
+    # RDF defines specific version strings, but does not define normative behavior
+    VERSIONS = %w{1.1 1.2-basic 1.2}.freeze
+
     ##
     # Enumerates known RDF serialization format classes.
     #