Update Node.js version in JS tutorial

[yukihirai0505]

If we use Node.js 8.7.0 version, we get following an error message.

error webpack-serve@2.0.3: The engine "node" is incompatible with this module. Expected version ">= 6.9.0 < 7.0.0 || >= 8.9.0". Got "8.7.0"
error Found incompatible module

It is better to update the document to avoid this error.

[yukihirai0505]

😥

[roadscape]

Thanks @yukihirai0505!