Don't try to make potions unbreakable.

The catch-all approach to unbreakability works in modern Minecraft
because it's just a flag. Historians speculate that back in ancient
times when Minecraft 1.8 was relevant and worth anyone's time, no such
flag existed, and as a result, primitive plugin developers of the ages
resorted to hacks where the durability was just set to something very
high (or low, rather), making stuff _effectively_ unbreakable. However,
because we didn't know better back then, we also used the durability to
configure potion metadata. As a result, making potions "unbreakable"
just means changing their type, and `Short.MIN_VALUE` is not a valid
potion type, so it just defaults to water bottle.

For the love of maintainability, please just let 1.8 die...

Fixes #633

src/main/java/com/garbagemule/MobArena/ArenaImpl.java

@@ -1267,6 +1267,9 @@ public class ArenaImpl implements Arena
         if (stack == null) {
             return;
         }
+        if (stack.getType() == Material.POTION) {
+            return;
+        }
         stack.setDurability(Short.MIN_VALUE);
     }