checkVersions on `nodewebkit` module info.

This fixes the checkVersions issue https://github.com/mllrsohn/node-webkit-builder/issues/9#issuecomment-41518785

Instead of parsing amazon's xml, it reads `nodewebkit` npm info package.

It works and it's simple. In the future, maybe @rogerwang will provide a parsable file of versions (https://github.com/rogerwang/node-webkit/issues/1821) to replace npm info, and the code should be fairly easy to adapt. 

I modified getVersionNames to also accept an array of versions, to avoid repetition of code.

Also simplified versions test