Fix generation of jungle trees. Fixes BUKKIT-5043

Due to changes in the generation of trees, the name of the class responsible
for the generation of jungle trees has changed from WorldGenMegaTree to
WorldGenJungleTree.  As such, references to WorldGenMegaTree need to be
updated to WorldGenJungleTree to generate the correct type of tree.

Pulled from PR #1277

src/main/java/org/bukkit/craftbukkit/CraftWorld.java

@@ -370,7 +370,7 @@ public class CraftWorld implements World {
             gen = new WorldGenTaiga1();
             break;
         case JUNGLE:
-            gen = new WorldGenMegaTree(true, rand.nextBoolean());
+            gen = new WorldGenJungleTree(true, 10, 20, 3, 3); // Magic values as in BlockSapling
             break;
         case SMALL_JUNGLE:
             gen = new WorldGenTrees(true, 4 + rand.nextInt(7), 3, 3, false);