Follow Jekyll 2.3.0 internal API change

Jekyll::Convertible#transform now returns a transformed content,
doesn't update self.content any longer.

Changed in jekyll/jekyll@d004bc4.

Author: yaegashi

lib/jekyll-haml/ext/convertible.rb

@@ -13,7 +13,7 @@ def initialize(site, base, name)
 
       self.process(name)
       self.read_yaml(base, name)
-      self.transform
+      self.content = self.transform
     end
   end
 end