CI/Appveyor: Appveyor breaks on old npm

[ryan-roemer]

See: https://ci.appveyor.com/project/FormidableLabs/builder/builds/23844480/job/n1ysvdon9ejkc4rj

Installing node 4.9.1 (x86)...
npm install -g "npm@$env:TEST_NPM_VERSION"
C:\Users\appveyor\AppData\Roaming\npm\npm -> C:\Users\appveyor\AppData\Roaming\npm\node_modules\npm\bin\npm-cli.js
C:\Users\appveyor\AppData\Roaming\npm\npx -> C:\Users\appveyor\AppData\Roaming\npm\node_modules\npm\bin\npx-cli.js
npm@6.9.0 C:\Users\appveyor\AppData\Roaming\npm\node_modules\npm
npm install
C:\Users\appveyor\AppData\Roaming\npm\node_modules\npm\bin\npm-cli.js:84
      let notifier = require('update-notifier')({pkg})
      ^^^
SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:373:25)
    at Object.Module._extensions..js (module.js:416:10)
    at Module.load (module.js:343:32)
    at Function.Module._load (module.js:300:12)
    at Function.Module.runMain (module.js:441:10)
    at startup (node.js:140:18)
    at node.js:1043:3
Command exited with code 1

Might just need to drop old node. Probably good opportunity to adjust to only 8, 10, 11