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/Add-More-Creeper-API.patch

@@ -65,7 +65,7 @@ index 00000000..ff10251b
 +    }
 +}
 diff --git a/src/main/java/org/bukkit/entity/Creeper.java b/src/main/java/org/bukkit/entity/Creeper.java
-index f957d836..b9877fb8 100644
+index 32f18a3a..601ba4af 100644
 --- a/src/main/java/org/bukkit/entity/Creeper.java
 +++ b/src/main/java/org/bukkit/entity/Creeper.java
 @@ -0,0 +0,0 @@ public interface Creeper extends Monster {