update to mongodb-native v1.4.4
note: bson need some fixture for aggregation
      
      bson.js@0.2.8 L:352
      // Check the key and throw error if it's illegal
      // 2014/05/18 [ystskm] remove check for aggregation, query
      /*
      if (key != '$db' && key != '$ref' && key != '$id') {
        // dollars and dots ok
        BSON.checkKey(key, !checkKeys);
      }*/