Drop Node.js 4 & other housekeeping chores

[novemberborn]

We're only a few weeks away from Node.js 4 reaching end-of-life. It's likely that Babel 7 will drop support, and it's also quite likely we won't ship the non-beta 1.0 release until after the cut-off. Consequently let's drop Node.js 4 now, rather than next month.

I've amended the support statement to allow for this circumstance:

We may drop support for a Node.js version, in a major-version-bumping-pre-release, if that new AVA version is expected to become stable around or after the end-of-life date of the Node.js version in question.

Let's hold off on aggressively rewriting code until the big 1.0 work completes.

Housekeeping wise I've upgraded npm to 5.8.0 so we can utilize the npm ci command. I'm still verifying the package-lock.json matches what npm in our CI environment generates, but the lock-verify step has been removed from our lint command.

Most dependencies have been updated, including TypeScript and Flow.

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I'm going to land this once CI passes, so I can rebase #1722 and land that one too. Happy to make changes in response to feedback in a follow-up PR.