buffer: rename internal/buffer_new.js to buffer.js

The old smalloc-based implementation has been removed, the typed array
implementation is the only one that is left now.

PR-URL: https://github.com/nodejs/io.js/pull/2022


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>