Clean up debug code
Fix bug with Ruby-2.0 and Ruby-1.9

Ruby >= 1.9 / exec has same options as (new) :spawn,
which closes all non-standard unspecified descriptors by default.
To keep the fds open, we need
- set flag :close_on_exec => false for the :fd
- use option :close_others => false when run :exec

Question: If a :fd has :close_on_exec => true, will :spawn/:exec
close that :fd when :close_others => false is specified?
Bump new version 1.1.5