Update to Minecraft 1.13.1

By: md_5 <git@md-5.net>

paper-server/src/test/java/org/bukkit/potion/PotionTest.java

@@ -1,11 +1,11 @@
 package org.bukkit.potion;
 
 import static org.junit.Assert.*;
-import static org.hamcrest.Matchers.*;
 
 import java.util.EnumMap;
 import java.util.List;
 import java.util.Map;
+import net.minecraft.server.IRegistry;
 
 import net.minecraft.server.MobEffect;
 import net.minecraft.server.MobEffectList;
@@ -17,7 +17,7 @@ public class PotionTest extends AbstractTestingBase {
     @Test
     public void testEffectCompleteness() throws Throwable {
         Map<PotionType, String> effects = new EnumMap(PotionType.class);
-        for (Object reg : PotionRegistry.a) {
+        for (Object reg : IRegistry.POTION) {
             List<MobEffect> eff = ((PotionRegistry)reg).a();
             if (eff.size() != 1) continue;
             int id = MobEffectList.getId(eff.get(0).getMobEffect());