Update package.json, removed zlib dependency

Hi there!

I recently ran into an npm install error with node-rest-client.
Since version 0.8.0 of node, it does not contian node-waf anymore. The node-zlib package which you make use of, depends on node-waf. Therefore I could not build node-rest-client with a decent version of nodejs (0.10.28 over here).

Since zlib is a core lib of nodejs now, we probably don't need external npm-zlib over here.

This worked well for me. Now it is up to your decision if you want to leave this one out or not. Just my 2 cents. ;)

Regards,
Dennis