chore: upgrade Node.js to 8

[DanielRuf]

Description

Node.s 6 a bit old (4 is EOL since last month), 8 is the current LTS release and 10 is the current stable. So we should check the current Node.js LTS release which will also bring a better performance (and also automatic npm 5 with automatic package-lock.json generation).

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[VladimirZaets]

@DanielRuf , thank you for collaboration. Good job)

[magento-engcom-team]

Hi @DanielRuf. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.2.5 release.

[DanielRuf]

I guess we should also do this in the 2.3-develop branch. Should / can I collect my PRs that I have to port to 2.3-develop in a separate issue as reminder for me?