Use the shared travis config with auto-updates for node releases

[dominykas]

tl;dr: I'd like to open a PR to replace the node_js section in the .travis.yml with an imported https://github.com/nodejs/ci-config-travis/blob/master/lts/gte-8.yml

Background: Package Maintenance WG has built a repository of shared Travis CI imports. The imports will be automatically updated as new Node.js versions get released.

Judging by what's inside the current .travis.yml here, I suspect that the lts upgrade policy is the right fit (it adds new versions as soon as they are released, never removes LTS versions, removes non-LTS versions when they reach EOL).

The engines field in the package.json should probably also get a bump (and a major release)? Alternatively, we can try to import the gte-4.yml config, to ensure it gets coverage.

#10 should probably get merged first, though.

Is that OK?