Respect TERM_CHILD env var

In resque-1.22 and above, the signals `TERM` and `QUIT` may be reversed to match POSIX by setting an env var `TERM_CHILD`.

This code change causes resque-pool to also respect the new semantics of `TERM` and `QUIT` when this variable is set.

In resque-2.0, the new POSIX-compliant signals will be the default, and I suspect this gem will want to mirror that signal-handling as well. So this code could also be useful when building a resque-2.0 version of rescue-pool.

Also contains a bonus cucumber fix for MacOS developers.