chore: clarify node_js versions in Travis config (#6570)
chore: clarify node_js versions in Travis config (#6570)

This patch makes it so that we only refer to specific Node.js versions like `10.19.0` when we really mean to use exactly that version. Usually we just want to test the latest version in a major range, e.g. `10`.

It also adds an inline comment that makes it easier to update the versions across the file in the future (e.g. when Node.js 10 goes EOL in April 2021).

Issue: #6561
puppeteer/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer