Fix NPM verification failures after `script/clean` on Win32

I think npm verification is failing unfairly on me when I try to build Atom on my Windows machine.

```
C:\Users\james_000\Documents\src\atom [atom-master]> .\script\bootstrap
npm 1.4 is required to build Atom. An error (Error: spawn ENOENT) occured when checking the version.
C:\Users\james_000\Documents\src\atom [atom-master]> npm -v
1.4.9
```

After a `script/clean` on my machine, there's no local copy of npm and therefore the npm verification script defaults to `npm`, which for some reason doesn't seem to resolve correctly to the `npm.cmd`. I'm not sure why but that seems to be the behavior I'm seeing.

Using my proposed changes I see:

```
C:\Users\james_000\Documents\src\atom [improve-npm-verify-on-win32 +0 ~1 -0]> .\script\bootstrap
Node: v0.10.28
npm: v1.4.9
Python: v2.7.6
```