update Node.js to 6.16+

[mcandre]

Resolve CI build errors by updating Node.js to match modern dependency requirements.

[dougwilson]

Thank you, but if you need something like this, I would suggest publishing a fork to npm that fits your needs. Most people who maintain module use them too, like I do. I still have some Node.js 4 deployments I look after, though we are working to move off of that version, but is difficult due to the legacy systems it is with.