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).
@tabolario - I think the easiest way is to set the exit code to a specific one during the SIGINT handler, then check the value here.