Add support for Elytra as chest pieces in class chests.

Makes the class chest armor guessing logic pair `"ELYTRA"` up with the
chest piece slot. This specific matching works, because there are no
underscores in the `"ELYTRA"` item name.

Fixes #616

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

@@ -1189,6 +1189,7 @@ public class ArenaImpl implements Arena
                 makeUnbreakable(stack);
             }
             switch (type) {
+                case "ELYTRA":
                 case "CHESTPLATE": chestplate = stack; break;
                 case "LEGGINGS":   leggings   = stack; break;
                 case "BOOTS":      boots      = stack; break;