Supported versions

A very good question brought up by on @ronkorving https://github.com/spdy-http2/node-spdy/pull/309#issuecomment-293491240

I'm interested to see what are the latest stats on [Node.js version usage](https://twitter.com/daviddias/status/852077404839391232). I believe we can have a good strategy for following Node.js LTS release cycle.

Looking forward to hearing everyone thoughts :)