Change check for Haml version.

Author: gkellogg

lib/rdf/vocab/extensions.rb

@@ -391,7 +391,7 @@ def to_html(graph: nil, prefixes: nil, jsonld: nil, template: nil)
         case template
         when /.haml$/
           require 'haml'
-          haml = if Haml.const_defined?(:Template)
+          haml = if Haml::VERSION >= "6"
             Haml::Template.new {File.read(template)}
           else
            Haml::Engine.new(File.read(template))