Fix test of silent exit on unsupported node.js versions

[voxpelli]

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update
[x] Bug fix
[ ] New feature
[ ] Other, please explain:

What changes did you make? (Give an overview)

• Used version-guard for the silent exit (this helped identify a mismatch in the version set in bin/cmd.js and the one in package.json)
• Used .cjs file ending to avoid having to resort to eval()
• Gathered all non-index.js files (since there's now a cli.js file as well) in lib/
• Added Node.js 4 to old-test.yml, to get a a signal of how far back any possible breakage goes
• Stopped caching dependencies as we don't really want these very old npm versions to mess around with the cache that's shared by the rest
• Stopped installing dev dependencies in old-test-yml to avoid problems on npm install (this fixed the tests)

Which issue (if any) does this pull request address?

Closes #1828

Is there anything you'd like reviewers to focus on?

[theoludwig]

Thanks for investigating how to fix the old tests on master.
Some comments need more clarification of what's going on.

While I still think, we should not do some "version guard" things, I don't mind for the moment if we continue to fail silently for the older Node.js versions. About that, I will continue to discuss that in #1828, so this PR only talks about the implementation, not the possible solutions. 😄

[theoludwig]

LGTM!