Versioning the buffertools Dependency

This was written against `buffertools` version 1, and there has been [at least one breaking change](https://github.com/bnoordhuis/node-buffertools/commit/20b192152d398b2d5b301f00d1e5daa22f6f2e23) since then.

I came across this issue when attempting to use [node-dir-diff](https://github.com/bingomanatee/node-dir-diff) in `content` mode to compare directories.