Add newer node versions to engines, CI

[AndrewADev]

While setting up a project using grunt-istanbul locally, I noticed an EBADENGINE warning from Node based on my current Node version:

It looks like at least the CI is testing with Node 4.1. Given the usage stats via GitHub, and consistent NPM downloads (as well as the fact that I'm seeing this in a project using Node 16), it seems likely that its being used successfully on a number of newer Node versions.

As such, would you be open to:

• Adjusting the specifier in engines to include newer node versions?
• Confirming these via new node versions in the Travis file?