Fixes context for notFound in `NpmProxy.decide`

If you were to enable whitelist and you try to install a package that is
not in the whitelist, an error is thrown when `this.log` is called in
`lib/npm-proxy.js` on line 244.  By referencing `this.notFound` on line
188, we lose `this` in `NpmProxy.notFound`
Merge pull request #1 from evanlucas/reference-issue

Fixes context for notFound in `NpmProxy.decide`