tests: Stop testing Node.js 4.

[abernix]

Node.js 9, as with odd numbered Node.js versions, is no longer longer being
actively maintained with the release of (even numbered) Node.js 10.

Node.js 4 will no longer be receiving any additional updates from the
Node.js foundation.

For more details see the LTS schedule and Node.js blog posts:

Ref: https://medium.com/the-node-js-collection/april-2018-release-updates-from-the-node-js-project-71687e1f7742
Ref: https://github.com/nodejs/LTS

[abernix]

It appears that npm@5 is currently complaining about Node.js 10. This will likely be fixed in an upcoming npm 5.9.x, but currently the Node.js 10 tests show:

npm WARN npm npm does not support Node.js v10.0.0
npm WARN npm You should probably upgrade to a newer version of node as we
npm WARN npm can't make any promises that npm will work with this version.
npm WARN npm Supported releases of Node.js are the latest release of 4, 6, 7, 8, 9.
npm WARN npm You can find the latest version at https://nodejs.org/

[abernix]

In order to move this along, I'll break the Node 9 to Node 10 change (a4a85bd) out into a separate PR. Removing the Node.js 4 tests is the most pressing since upstream libraries are also following that trend and new PRs are being opened that are still testing Node.js 4 (and only those tests are failing).

(And yes, the Node.js 4 tests will never be reported on this PR since they are not being run at all — I'll update the repository settings after merging this with administrator privileges.)