Fixed AI constantly visiting towns thinking they can get a huge upgrade

Due to morale-considerations the AI sometimes calculated that their strongest army after doing an exchange had slightly lower total value than the army they used before.
But by using unsigned "slightly lower" became near infinite.
So they constantly wanted to upgrade their army because they considered it more useful than anything else.
Changing the unsigned into signed fixes this.

AI/Nullkiller/Analyzers/ArmyManager.h

@@ -32,7 +32,7 @@ struct SlotInfo
 struct ArmyUpgradeInfo
 {
 	std::vector<SlotInfo> resultingArmy;
-	uint64_t upgradeValue = 0;
+	int64_t upgradeValue = 0;
 	TResources upgradeCost;
 
 	void addArmyToBuy(std::vector<SlotInfo> army);