I'm not sure what's going on, but it started happening to me locally first.
Runin ember t would cause phantomjs to crash after the 7th or 8th test module (not sure which at the moment). Deleting node_modules and rerunning npm install fixes it locally. Pushing to the remote then causes Circle CI to fail for the same reason, so a Rebuild without cache is needed. After that, everything starts working again.