I found the answer for anyone interested.

In the http://AppController.mm

Look for the function:

- (void) applicationDidReceiveMemoryWarning:(UIApplication*)application
{
 printf_console("WARNING -> applicationDidReceiveMemoryWarning()\n");
}

This is the function that is called when your app receives a low memory warning.

Next you must add a UnitySendMessage to your Unity code, so that you can receive the low memory warning message in your Unity code.

So, you must modify the applicationDidReceiveMemoryWarning function by writing this code:

- (void) applicationDidReceiveMemoryWarning:(UIApplication*)application
{
 printf_console("WARNING -> applicationDidReceiveMemoryWarning()\n");

    // This is your added call to send a message to your Unity code.
    // The first parameter is the name of the class you'll be using to receive the
    // low memory warning message. Mine is MemoryManager, yours can be whatever
    // you name your class.
    // The second parameter is the name of the method/function within your class
    // that you will be using to receive the low memory warning and handle it 
    // appropriately however you see fit. Mine is ReceivedMemoryWarning, yours can
    // be whatever you like.
    // Don't forget that your function/method within your Unity C#/Javascript/etc
    // code, must be a void return value with a string parameter.
    // E.g. 
    // public void ReceivedMemoryWarning(string message)
    // The third parameter can be a null string, or whatever string you want your 
    // message that is sent to say.

    UnitySendMessage("MemoryManager", "ReceivedMemoryWarning","");
}

After you add this message call to the http://AppController.mm file, in your Unity code, add a new file for the class that will be receiving the message. Mine happens to be in C#, but it can be any file type. Mine is MemoyManager.cs.

Then add the code:

using UnityEngine;
using System.Collections;

public class MemoryManager : MonoBehaviour
{
 //Function is called when a lowMemoryWarning is received
 public void ReceivedMemoryWarning(string message)
 {
 Debug.Log("Memory Manager RECEIVED LOW MEMORY WARNING!");
 }
}

This class now receives the message sent from the http://AppController.mm, and within your own function, mine is ReceivedMemoryWarning, handle how you deal with, free up memory within your game/project.

Also, it's worth mentioning if you want to modify the http://AppController.mm and have it update automatically in xCode every time, first create a folder in your Assets folder within the Project hierarchy window named "iOS". Then drag the http://AppController.mm file into that folder with your modified code, and Unity will automatically update your changes when it builds to xCode.

I hope that helps for everyone who is looking to do this, whether you're a programmer or not.