Flush stdout/stderr before calling `process.exit()`

Let me know what you think. I tried to follow your coding style: semi-colonless! =)

This is essentially the same fix that's been suggested on https://github.com/joyent/node/issues/3871 and had been applied to other libs like jshint, r.js, etc. so I'm not reinventing the wheel.

I verified that the output is no longer truncated when running `testem ci` in a subprocess. And both standalone `testem` and `testem ci` continue to work as they should.