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/0116-Add-World.getEntity-UUID-API.patch

@@ -0,0 +1,31 @@
+From 30e95a4cbd997640b4ad3b63ae46e8b8caf15161 Mon Sep 17 00:00:00 2001
+From: Brokkonaut <hannos17@gmx.de>
+Date: Tue, 3 Jul 2018 16:07:16 +0200
+Subject: [PATCH] Add World.getEntity(UUID) API
+
+
+diff --git a/src/main/java/org/bukkit/World.java b/src/main/java/org/bukkit/World.java
+index 8f69dced..0b328315 100644
+--- a/src/main/java/org/bukkit/World.java
++++ b/src/main/java/org/bukkit/World.java
+@@ -776,6 +776,17 @@ public interface World extends PluginMessageRecipient, Metadatable {
+     @NotNull
+     public Collection<Entity> getNearbyEntities(@NotNull Location location, double x, double y, double z);
+ 
++    // Paper start - getEntity by UUID API
++    /**
++     * Gets an entity in this world by its UUID
++     *
++     * @param uuid the UUID of the entity
++     * @return the entity with the given UUID, or null if it isn't found
++     */
++    @Nullable
++    public Entity getEntity(@NotNull UUID uuid);
++    // Paper end
++
+     /**
+      * Returns a list of entities within a bounding box centered around a
+      * Location.
+-- 
+2.21.0
+