#261 is seems parameters aren't defined on the response class

Probably this was the reason it was previously returning null
I couldn't see a clear case for starting to store parameters as a property so went with the previous person's analysis

Author: eileenmcnaughton

src/Omnipay/Common/Message/AbstractResponse.php

@@ -144,7 +144,7 @@ public function getCode()
      */
     public function getTransactionReference()
     {
-        return $this->getParameter('transactionReference');
+        return null;
     }
 
     /**
@@ -154,7 +154,7 @@ public function getTransactionReference()
      */
     public function getTransactionId()
     {
-        $this->getParameter('transactionId');
+        return null;
     }
 
     /**