TL;DR: If this is happening to you, just add the line sudo:required to travis.yml.
After a suitable amount of pulling of hair (my own), gnashing of teeth, and general fretting, I have found a solution to a rather nettlesome issue. I have some elm code in a project that uses webpack and Travis CI. It all works fine locally, but when I tried to build it on Travis, the webpack command apparently hung without any warnings or other clues.
At first I guessed that the problem might be the fact that by default, at the time of this problem, Travis was setting me up with Ubuntu 14.04. This being a rather long-in-tooth edition by now, perhaps that's a problem. My research into how-to-specify a more recent version of Ubuntu was discouraging because apparently that was the latest edition available. I was welcome to redesign the entire CI process to use docker instead, but that sounded like work, so I contemplated further.
I next figured that perhaps the problem was between webpack and Travis. I found that if