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).
This pattern is used in a few places, and I think we should have a more specific exit code for `SIGINT` in general so that we can check it from these sorts of acceptance tests. 

Merging as is for now, and will open another issue to track the fixing of the `SIGINT` and `SIGTERM`.