Support Node.js >= 4.0.0

[TimothyGu]

Fixes #58.

[bluedreamfly]

After you fix bug, I have found a problem that node4.0.0 have no corresponding npm version,so when I run nvmw install 4.0.0,It have a error that have no npm from site "var NPM_PKG_JSON_URL = 'https://raw.githubusercontent.com/%s/%s/deps/npm/package.json';" your get package.json

[ChALkeR]

Adding to the list: https://github.com/nodejs/node/issues/2798.