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/0154-Material-API-additions.patch

@@ -0,0 +1,44 @@
+From ca41b95550077f665eafda0196e09f76153f6a86 Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Sat, 6 Oct 2018 21:14:29 -0400
+Subject: [PATCH] Material API additions
+
+
+diff --git a/src/main/java/org/bukkit/Material.java b/src/main/java/org/bukkit/Material.java
+index b9838dc9..9a1c3724 100644
+--- a/src/main/java/org/bukkit/Material.java
++++ b/src/main/java/org/bukkit/Material.java
+@@ -80,6 +80,7 @@ import org.jetbrains.annotations.Nullable;
+ /**
+  * An enum of all material IDs accepted by the official server and client
+  */
++@SuppressWarnings({"DeprecatedIsStillUsed", "deprecation"}) // Paper
+ public enum Material implements Keyed {
+     //<editor-fold desc="Materials" defaultstate="collapsed">
+     ACACIA_BOAT(27326, 1),
+@@ -3291,6 +3292,22 @@ public enum Material implements Keyed {
+         }
+     }
+ 
++    // Paper start
++
++    /**
++     * @return If the type is either AIR, CAVE_AIR or VOID_AIR
++     */
++    public boolean isEmpty() {
++        switch (this) {
++            case AIR:
++            case CAVE_AIR:
++            case VOID_AIR:
++                return true;
++        }
++        return false;
++    }
++    // Paper end
++
+     /**
+      * Do not use for any reason.
+      *
+-- 
+2.21.0
+