Don't automatically close the shared pipe used to get the DRb URI.

Ruby 2.0+ automatically closes all descriptors except 0, 1, 2 (input/output/error)
in child processes.  We need the child process (the server) to write it's DRb URI to
the writer descriptor so the client can read it.  Set close_on_exec to false
in the writer descriptor so it doesn't get closed on us.
See: https://bugs.ruby-lang.org/issues/5041

Fixes #459