Use `Buffer.from` instead of `new Buffer`

[ericyd]

new Buffer has been deprecated since Node v6.0.0

When this lib is bundled with zero-dependencies, e.g. in a serverless function like AWS lambda, this code emits a deprecation warning.

Not a huge deal but figured it might be nice to update it.

Buffer.from was added in Node v5.10.0 so this probably counts as a major version bump since it will break apps running Node < v5.10.0