only enable source annotation on rails >= 4.2

Rails 3.2 .. 4.1 already supported HAML. Also, the original previous
commit would have broken on Rails < 4.2.

Author: jparker

lib/haml-rails.rb

@@ -47,8 +47,12 @@ class Railtie < ::Rails::Railtie
 
       initializer 'haml_rails.configure_source_annotation' do
         # Configure source annoatation on haml files
-        SourceAnnotationExtractor::Annotation.register_extensions('haml') do |tag|
-          /\s*-#\s*(#{tag}):?\s*(.*)/
+
+        # HAML support existed in Rails 3.2, 4.0, and 4.1, but was dropped in 4.2
+        if ::Rails.version >= '4.2'
+          SourceAnnotationExtractor::Annotation.register_extensions('haml') do |tag|
+            /\s*-#\s*(#{tag}):?\s*(.*)/
+          end
         end
       end
     end