protected methods return false unless a second argument is passed to

respond_to?.  This only impacts trunk ruby, but will be included in Ruby
2.0.

  https://bugs.ruby-lang.org/projects/ruby-trunk/repository/revisions/34580

lib/sprockets/directive_processor.rb

@@ -126,7 +126,7 @@ def directives
       @directives ||= header.lines.each_with_index.map { |line, index|
         if directive = line[DIRECTIVE_PATTERN, 1]
           name, *args = Shellwords.shellwords(directive)
-          if respond_to?("process_#{name}_directive")
+          if respond_to?("process_#{name}_directive", true)
             [index + 1, name, *args]
           end
         end