Node 8 support

[ehmicky]

Our Node.js support is currently >=8.

This plugin is using fs.promises, which was introduced in Node 10. Using require('util').promisify(fs.appendFile) instead would make it work for Node 8 users.

Also, do you know whether next-on-netlify works on Node 8?

It'd be nice to add some engines.node like this. Our build system detects this property and prints a warning message when a site is using a Node.js version incompatible with a plugin.

Finally, our CI tests should run on Node 8 to ensure support.

[ehmicky]

Next.js 9 only supports Node.js >=10.13.0:

https://github.com/vercel/next.js/blob/118ab7992bc8f7a7e5a7bb996510d9b56ffe4f68/package.json#L137

https://nextjs.org/docs/getting-started#system-requirements

So, unless we want to support Next.js 8, we will need to enable this plugin only if the build's Node.js version is >=10.13.0.

[ehmicky]

Since we only support Next.js >=9.5.3 (see #25), we can limit support for Node.js >=10.13.0.

[ehmicky]

Fixed by #41.