runner: check exit_exec on for OS X jobs
runner: check exit_exec on for OS X jobs

Not including this caused `stop`s `session.close` to close the session
while the script was still running, which then makes the session close
to be marked as unexpected and the job is requeued. This means that
cancellations don't work right and cause "infinite" requeues.

travis-ci/travis-worker,travis-ci/travis-worker,ireznice/travis-worker,gavioto/travis-worker,gavioto/travis-worker,final-ci/travis-worker,final-ci/travis-worker,ireznice/travis-worker