Fixes issue where players lost a box size increment.

All boxes start with a minimum size of 1. This was not being included in
the box size calculation, so players were erroneously losing one box
size when they logged in or teleported.

src/main/java/world/bentobox/boxed/AdvancementsManager.java

@@ -150,7 +150,8 @@ public boolean hasAdvancement(Island island, String advancement) {
      * @return value of island size change. Negative values means the island range shrank.
      */
     public int checkIslandSize(Island island) {
-        int shouldSize = getIsland(island).getAdvancements().stream().mapToInt(this::getScore).sum();
+        // Island is always a minimum of 1 for free.
+        int shouldSize = getIsland(island).getAdvancements().stream().mapToInt(this::getScore).sum() + 1;
         if (shouldSize < 1) {
             // Boxes can never be less than 1 in protection size
             return 0;