Stop allowing everyone to change the instance

src/main/java/us/myles/ViaVersion/api/ViaVersion.java

@@ -1,6 +1,7 @@
 package us.myles.ViaVersion.api;
 
 import lombok.Getter;
+import org.apache.commons.lang.Validate;
 import us.myles.ViaVersion.ViaVersionPlugin;
 
 public class ViaVersion {
@@ -11,6 +12,7 @@ public class ViaVersion {
     private static ViaVersionConfig config;
 
     public static void setInstance(ViaVersionPlugin plugin) {
+        Validate.isTrue(instance == null, "Instance is already set");
         ViaVersion.instance = plugin;
         ViaVersion.config = plugin;
     }