Upgrade to Yarn 1.x across our build images, documentation, and GDK scripts

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Yarn 1.0 is now out, and it contains several large improvements and bug fixes for issues we've been occasionally running into in our CI process. Now that it has reached 1.x and is now "stable" we ought to enforce this version as much as we can.

I suggest:

• updating our build and CI images (gitlab-build-images#14 (closed))
• add a version check to the GDK so our devs are reminded to upgrade
• updating our documentation to recommend yarn 1.0 or later