use node 12 in CI
use node 12 in CI

Summary:
Pull Request resolved: https://github.com/facebook/flow/pull/8740

node 12 is the oldest version that's still maintained, so that's the oldest we are going to target.

Changelog: [internal]

Reviewed By: gkz

Differential Revision: D30609813

fbshipit-source-id: 73b0aece362a50941fc1f7f42d289c953a1ce191

mroch/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow