Several upgrades and improvements to CI

[tripu]

Looking at #217 I noticed that there was no info on the PR about Travis builds and the results of tests. I think that's because we're still on the (now deprecated) travis-ci.org, instead of the newer travis-ci.com.

I fixed that, plus a few other old shortcomings:

• travis-ci.org → travis-ci.com
• On Travis, build with latest Ubuntu (Focal)
• On Travis, build with both Node.js 16 & 18, and latest npm
• Use npm ci instead of npm i; add the build step to the CI cycle, too
• Specify these new Node.js & npm requirements in README and enforce them in field engines of package.json
• Rebuild lock file with modern npm ("lockfileVersion": 2)
• Move default branch to master
• Upgrade all upgradeable devDependencies

After merging this, I'd move the default branch here from gh-pages to master (the only reason it was gh-pages was that at the time that was the only branch that GH Pages was able to serve, but that has changed now).

[tripu]

wave @tripu I might be missing something but should Travis be running against this branch? I don't see the status of tests against the PR.

I set up the Travis config so that only master is built. To save resources and avoid builds in all branches. But we can remove those two lines and have builds of everything, that's fine by me!

[tripu]

Thanks for reviewing, @tristen.

Can you move the default branch from gh-pages to master now, and set up GH Pages so that it serves what's in master instead? :pray:

[tripu]

Thank you, @tristen! :partying_face: