close open4 streams to prevent zombies

The popen4 that execs pygmetize leaves a bunch of zombie processes
around unless these streams are closed for some reason. It's not too
bad when running jekyll in one-shot mode but when running with
--server --auto --pygmentize, I eventually get "fork: resource not
available" errors :/ Closing the streams let's the processes die
while the parent stays running.

lib/jekyll/albino.rb

@@ -65,6 +65,7 @@ class Albino
       stdin.puts @target
       stdin.close
       output = stdout.read.strip
+      [stdout, stderr].each { |io| io.close }
     end
     output
   end