Fix categories filter

_plugins/categories.rb

@@ -1,5 +1,5 @@
 module Jekyll
-  module Filters
+  module CategoriesFilter
     # Outputs a list of categories as comma-separated <a> links. This is used
     # to output the category list for each post on a category page.
     #
@@ -12,3 +12,5 @@ module Jekyll
     end
   end
 end
+
+Liquid::Template.register_filter(Jekyll::CategoriesFilter)