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-Server-Patches/0047-Expose-server-CommandMap.patch

@@ -0,0 +1,21 @@
+From a9f87d27fac27702fbbc8cf0b84786be08c7841b Mon Sep 17 00:00:00 2001
+From: kashike <kashike@vq.lc>
+Date: Thu, 3 Mar 2016 02:15:57 -0600
+Subject: [PATCH] Expose server CommandMap
+
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/CraftServer.java b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+index a29b7b792..00b8fcb30 100644
+--- a/src/main/java/org/bukkit/craftbukkit/CraftServer.java
++++ b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+@@ -1582,6 +1582,7 @@ public final class CraftServer implements Server {
+         return helpMap;
+     }
+ 
++    @Override // Paper - add override
+     public SimpleCommandMap getCommandMap() {
+         return commandMap;
+     }
+-- 
+2.21.0
+