Update knownBroken version

[ljharb]

As I've documented here in nvm install-latest-npm, the last version of npm that works on node < v6 is npm v5 - npm v6+ requires node 6.

[zkat]

What breaks with npm@6?

[ljharb]

Unfortunately I didn't document it in my commit message, but local testing indicates:

nvm install 5
npm install -g npm@6.0.0
…
$ npm ls
$HOME/.nvm/versions/node/v5.11.1/lib/node_modules/npm/bin/npm-cli.js:79
      let notifier = require('update-notifier')({pkg})
      ^^^

SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode

Adding 'use strict' to all files would address this as well.

[zkat]

Sounds like it breaks after all! Thanks for the PR, we'll be including it in a release soon. 👍 🐑 🚀 💥