Check if there's already a TTL index for the 'deleted' field. If there is, and the TTL value is different, delete it so it can be recreated by the mongodb-queue module
function listIndexes(callback) {
            debug('[%s] list existing indexes', self.queueName);
            collection.indexInformation({full: true}, function(err, indexInfo) {
              if (err) {
                debug('[%s] error getting indexInfo. skipping ttl index check: %s', self.queueName, err);
                return callback(null, null);
              }
              return callback(null, indexInfo);
            });
          }