Travis test failures

[danmactough]

We're seeing some bogus test failures being reported from Travis. For example: https://travis-ci.com/mapbox/underreact/jobs/262756120

Note: this is bogus because that job is intended to run only on the underreact subset of tests (which pass ✅), not the babel-preset-mapbox subset of tests (which fail in this job, but which have their own job that succeeds 🤷‍♂).