Add deprecation check for new Buffer

[zackschuster]

The new Buffer pattern has been deprecated since v6.0.0 of node. If you're on that version or later, the proposed code will use Buffer.from instead.

[lemmabit]

I don't like the look of the version sniffing or the duplicated code. There's a ponyfill for Buffer.from called buffer-from on npm—no one else uses it for some reason, but it looks legit. I'll use that. Thanks for the heads-up.