Check support for NodeJS 7/8

[bdbch]

A developer had this feedback on reddit:

Got this prompt when running yarn build on the starter theme:
yarn start v0.24.6 $ check-node-version --package && gulp node: 8.1.0 npm: 5.0.3 yarn: 0.24.6 Error: Wanted node version 6.0.0 (>=6.0.0 <7.0.0) To install node, run nvm install ^6.0.0 or see https://nodejs.org/ error Command failed with exit code 1.
I downgraded to node 6.11.0 to try the theme out but am wondering if Flynt will support version 8+ in the future.

I think we should either add a note to the documentation or make Node 8 work with Flynt