sass renderer shouldn't explode if haml isn't available

middleman-core/lib/middleman-core/renderers/sass.rb

@@ -95,7 +95,11 @@ module Middleman
         # Change Sass path, for url functions, to the build folder if we're building
         # @return [Hash]
         def sass_options
-          ctx = ::Middleman::Renderers::Haml.last_haml_scope || @context
+          ctx = if defined?(::Middleman::Renderers::Haml)
+            ::Middleman::Renderers::Haml.last_haml_scope || @context
+          else
+            @context
+          end
 
           more_opts = {
             load_paths: ctx.config[:sass_assets_paths],