Update to Yarn 1.0.1

[jchv]

Looks like Yarn 1.0 was released yesterday. Should we update the Dockerfiles to use 1.0.1? If so, which Dockerfiles? (I assume only the Node 8.4 ones?) I can make a PR, but I wanted to check what the temperature reading was (especially for older versions, like the Node 6 images.)

[jchv]

OK, nevermind. Just read the Yarn update policy.