chore: add Node.js 14 to CI

[dhmlau]

Related to https://github.com/strongloop/loopback-next/issues/6022

Add Node.js 14 to .travis.yml.

Checklist

👉 Read and sign the CLA (Contributor License Agreement) 👈

• [ ] npm test passes on your machine
• [ ] New tests added or existing tests modified to cover all changes
• [x] Code conforms with the style guide
• [x] Commit messages are following our guidelines

[dhmlau]

@slnode test please

[dhmlau]

@slnode test please

[dhmlau]

Force merging it as the machine for running CI on Windows is not available. The tests on Node.js 14 on travis passed.