Upgrade mongoskin version

Consider upgrading version of mongoskin dependency to resolve a memory related bug on ARM architecture in ext/bson (mongodb). 

Found and resolved by @davidgraeff . For more information check https://github.com/mongodb/js-bson/issues/37 and http://stackoverflow.com/questions/16746134/bus-error-on-mongodb-mongoclient-connect-for-raspberry-pi-arm

I've run the test cases (make test) after upgrading and all passed. I have this change running in production on a pi as we speak, I'll report back in case of errors.