dont keep a local cache

This is a fix for https://github.com/heroku/semver.io/issues/6

The `/cache/node.html` file in this repo was a relic from the days before the [node-version-resolver](https://www.npmjs.org/package/node-version-resolver) node module existed. This patch removes the old cache file and uses an updated version of node-version-resolver with a `__dirname` path fix.