Don't include logo in npm package

Ignoring this file/ not including it in the generated npm package makes sense, since they are being served through GitHub's CDN anyway. It seems like a micro optimization, but considering the long list of dependencies some modules have, it is generally speaking a good convention.

How it can be verified that it works:
* `npm pack`
* `tar -tf hapi-6.0.2.tgz` -> images directory no longer included

Alternative: Moving assets in separate gh-pages branch.