Move version command update checking to the implementation

This makes it easier for downstream projects (forks) to replace the
version fetching system with their own. It is as simple as implementing
an interface and overriding the default implementation of
org.bukkit.UnsafeValues#getVersionFetcher()

It also makes it easier for us to organize things like the version
history feature.

Lastly I have updated the paper implementation to check against the site
API rather than against jenkins.

Spigot-API-Patches/0174-Entity-getEntitySpawnReason.patch

@@ -0,0 +1,30 @@
+From 6eb4f8f6f02a2a720d97c5925953c48b247674eb Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Sun, 24 Mar 2019 00:21:23 -0400
+Subject: [PATCH] Entity#getEntitySpawnReason
+
+Allows you to return the SpawnReason for why an Entity Spawned
+
+Pre existing entities will return NATURAL if it was a non
+persistenting Living Entity, SPAWNER for spawners,
+or DEFAULT since data was not stored.
+
+diff --git a/src/main/java/org/bukkit/entity/Entity.java b/src/main/java/org/bukkit/entity/Entity.java
+index f41243f9..29229c64 100644
+--- a/src/main/java/org/bukkit/entity/Entity.java
++++ b/src/main/java/org/bukkit/entity/Entity.java
+@@ -657,5 +657,11 @@ public interface Entity extends Metadatable, CommandSender, Nameable, Persistent
+      */
+     @NotNull
+     Chunk getChunk();
++
++    /**
++     * @return The {@link org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason} that spawned this entity.
++     */
++    @NotNull
++    org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason getEntitySpawnReason();
+     // Paper end
+ }
+-- 
+2.21.0
+