Test and specify supported node versions

[asvae]

You can use nvm to speed up the process.

This is important because it could be very confusing for user if something doesn't work for them out of the box.

• [ ] Add node version to readme file.
• [ ] Add node version to package lock.
• [ ] Add .nvmrc to project root with node version

Should be done after https://github.com/epicmaxco/vuestic-admin/issues/825 (as other packages are likely to impact end result)