fix for leaked file descriptors on Ruby 1.8.7

Util::Subprocess::POSIX leaked FDs on exec on Ruby 1.8.7 due to this issue:
https://bugs.ruby-lang.org/issues/5041.
fix for more leaked file descriptors

Closing FDs on the read side of pipes after subprocess execution.
Merge pull request #180 from jeremyasher/fd-leak-fixes

FD leak fixes