MongoDB depedency is breaking cluster

[drummerv]

Found a fix that works on Meteor 1.8:

Use a different version of Meteor's mongodb fork in https://github.com/meteorhacks/cluster/blob/master/package.js

I used "mongodb": "https://github.com/meteor/node-mongodb-native/tarball/1.3.7-with-null-checks" and it works normally now.