The travis image is installing chrome stable, which is not 59; AFAIK the headless feature is only available in beta right now which versions out to 59.
I admit I'm not sure if your unit tests are running in travis, but if they are, I'd wonder if the --headless flag is just getting dropped as an unknown flag.
Curious either way actually so let me know; I'm looking at starting the testing moving from phantomjs to headless which is why I was sleuthing out Clarity's move
Headless is pretty stable on Linux in 58. 59 is really when headless became usable on Mac. Still in beta on Windows.