Update Node.js version matrix

[nhardy]

Node.js 8.x is EOL as of 31/12/2019, and 13.x is now the current release, as per https://github.com/nodejs/Release

Is there a way to pull in all current/LTS versions rather than manually specifying these versions?

[nhardy]

Partially updated as part of #55

[nhardy]

Updated again as part of #112