Our CI is failing

[gaearon]

Likely cause: yarnpkg/yarn#2142.
We could hardcode Yarn 0.17 in the e2e test for now.

[gaearon]

#1167 fixes Yarn on Node 4 but Node 6 still fails although with a different error. FML.
https://travis-ci.org/facebookincubator/create-react-app/jobs/181468276