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).
Would a good way to do that perhaps be to have a failing change (e.g. syntax error) as the first build, which is removed and replaced with a successful one on the second one?