Consider forcing 5.x branch of node with npm 3.x

[justinhelmer]

Because this is such a high-level utility, it has installed modules that require working with files relative to both the install directory, and the current working directory.

Because of the differences in npm 2.x and npm 3.x, this package assumes npm 2.x is installed (by restricting node to <5.

Since this isn't something that needs integrating into other projects, it can arguably just require the latest version of node.