Update util.rb

A per comment (#39 (comment)) in PR(#39)

Author: adamboutcher

lib/puppet/util.rb

@@ -479,9 +479,9 @@ def safe_posix_fork(stdin = $stdin, stdout = $stdout, stderr = $stderr, &block)
 
       begin
         Dir.foreach('/proc/self/fd') do |f|
-          if f != '.' && f != '..' && f.to_i >= 3
+          if %{^\d+$}.match?(f) && f.to_i >= 3
             begin
-              IO.new(f).close
+              IO.new(f.to_i).close
             rescue
               nil
             end