Close #1197 ember build --watch should run until SIGTERM

Fixes #1197

Ensures that `ember build --watch` continues to run until SIGTERM (as per suggestion by @stefanpenner in https://github.com/stefanpenner/ember-cli/issues/1197).
Thanks! Sorry I didn't get a chance to play with this more on Sunday for that test. I'll keep an eye on the SIGINT/SIGTERM issue to see what I might be able to contribute there.