support auto detect version of npm

Change source of npm version definition from npm.json to `https://raw.github.com/joyent/node/vX.X.X/deps/npm/package.json.`

ex.
https://raw.github.com/joyent/node/v0.10.4/deps/npm/package.json
detect npm version is 1.2.18 for node v0.10.4.