Make `npm run shrinkwrap` work on Windows too

Maybe things will be OK with July's Windows 10 update, but until then on Windows mv doesn't exist.

cp and rm doesn't exist either so I can't think of a native workaround.

So I can only think of these solutions:

1. npm shrinkwrap accepts a path; I doubt this is reported or if so, it will change
2. save npm-shrinkwrap.json in root
3. try to find a grunt or a shell-js solution

/CC @cvrebert