Switched back from open4 to IO.popen.

open4 was added as a dependency when I was trying to parse STDERR and needed to work with STDIN. As it turns out, $stdin is good enough.  Redirecting STDERR to STDOUT seems to be good enough as well.  While open4 is probably still a cleaner approach, its reliance on fork() means it doesn't work on non-JRuby in Windows (echnically it doesn't work in JRuby either, but JRuby has an IO.popen4 special method that does virtually the same thing and we were using that instead).
Ruby 1.8.7 requires explicit requiring of 'thread' in order to use Queue.