Report test coverage using nyc

[dhui]

https://istanbul.js.org/docs/tutorials/mocha/

[dhui]

Looks like the build for node 4 is failing. Do we need to continue supporting node 4?

[jhermsmeier]

Fist of all, thank you for the work you've put in!
With the fairly minimal size of this module I have a hard time seeing the benefits of reporting test coverage though. Further, this breaks the Node v4 build due to nyc's incompatibility.
While there's no real reason to keep supporting Node v4, it'd have to be a major version bump to discontinue support for it, and there's also no reason to drop Node v4 support either.
So I'll close this for now, possibly to be revisited at a later time.
If you have some strong arguments in its favour, I'd love to hear them though.

[dhui]

Test coverage stats may have been useful in catching the broken has() implementation.

[dhui]

I've downgraded to a version of nyc that supports node v4: https://travis-ci.org/dhui/node-http-link-header/builds/464138253