Extend the Buffer prototype with buffertools.

A recent change to buffertools modified the default behavior such that the `Buffer` prototype is *no longer* extended by default.

This PR ensures the `Buffer` prototype is extended in `index.js`.

See https://github.com/bnoordhuis/node-buffertools/commit/20b192152d398b2d5b301f00d1e5daa22f6f2e23.