fix the use of Buffer in wget.js

This patch fixes the following error:

    C:\Users\koichik>nvmw install v0.11.3
    Start installing Node v0.11.3
    Microsoft (R) Windows Script Host Version 5.8
    Copyright (C) Microsoft Corporation 1996-2001. All rights reserved.

    Download from http://nodejs.org/dist/v0.11.3/node.exe, and save it as C:\Users\k
    oichik\git\hakobera\nvmw\v0.11.3\node.exe
    Done
    Start install npm
    Download file from https://raw.github.com/joyent/node/v0.11.3/deps/npm/package.j
    son
    package.json
    Content length is 3027
    Download 0 bytes ...
    Download 1024 bytes ...
    Download 2048 bytes ...
    Donwload done

    undefined:1

    ^
    SyntaxError: Unexpected token