npm 5, even the version included in the latest Node.js 8.5.0 re-generates
`package-lock.json` on each install. And when it does on a system that doesn't
support all the optional dependencies that are supported on the OS where the
lockfile was generated, it removes those optional deps from the lockfile.

The effect is that everyone firing `npm install` on our repo on any OS other
than macOS will immediately get a dirty state of the repo as the `fsevents`
dependency subtree gets removed from `package-lock.json`. That's a really bad
experience.

This commit removes package-lock.json from the repository and adds it to
.gitignore. We'll start committing the file again once the issue is resolved
on npm's part.

Fixes jquerygh-3792