make ref_id nullable

Hi,

According to the error and PoolPort source and the mentioned function, ref_id should be nullable :

Error:
    SQLSTATE[HY000]: General error: 1364 Field 'ref_id' doesn't have a default value (SQL: insert into `gateway_transactions` (`id`, `port`, `price`, `status`, `ip`, `created_at`, `updated_at`) values (1477224769, MELLAT, 1000, INIT, 192.168.10.1, 2016-10-23 15:42:50, 2016-10-23 15:42:50))

Insert function :
    protected function newTransaction()
	{
		$uid = $this->getTimeId();
		$this->transactionId = $this->getTable()->insert([
			'id' => $uid,
			'port' => $this->getPortName(),
			'price' => $this->amount,
			'status' => Enum::TRANSACTION_INIT,
			'ip' => Request::getClientIp(),
			'created_at' => Carbon::now(),
			'updated_at' => Carbon::now(),
		]) ? $uid : null;

		return $this->transactionId;
	}

poolport source includes this line : 

    `ref_id` varchar(255) COLLATE utf8_persian_ci DEFAULT NULL,

migrations/2016_05_02_193213_create_gateway_transactions_table.php

@@ -33,7 +33,7 @@ public function up()
 				Enum::ZARINPAL,
 			]);
 			$table->decimal('price', 15, 2);
-			$table->string('ref_id', 100);
+			$table->string('ref_id', 100)->nullable();
 			$table->string('tracking_code', 50)->nullable();
 			$table->string('card_number', 50)->nullable();
 			$table->enum('status', [