fix: add node version restriction

[stefanjudis]

When running axe-cli in in node version smaller than 6 (this is a least what I tested) it throws the following error:

> axe https://www.stefanjudis.de
/Users/stefanjudis/.nvm/versions/node/v5.12.0/lib/node_modules/axe-cli/node_modules/selenium-webdriver/index.js:115
  static createSession(...args) {}
                       ^^^

SyntaxError: Unexpected token ...
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:387:25)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Module.require (module.js:367:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/Users/stefanjudis/.nvm/versions/node/v5.12.0/lib/node_modules/axe-cli/lib/axe-test-urls.js:3:19)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)

To help future users it might make sense to define a proper version, so that npm could potentially warn the user.

[CLAassistant]

All committers have signed the CLA.

[marcysutton]

We do mention it in the README. It doesn't really warn users when they install it though, only if axe-cli is included as a dependency.

From npm:

Unless the user has set the engine-strict config flag, this field is advisory only will produce warnings when your package is installed as a dependency.

We also recommend installing axe-cli globally, so I'm not sure how useful this is. Probably doesn't hurt, though.

[stefanjudis]

Whooops. Yeah, didn't read that, sorry. :)

But yeah - won't hurt I guess. Thanks for merging.