Fix checkpoint error condition

SpigotCore_Main/src/de/steamwar/core/CheckpointUtils.java

@@ -78,7 +78,7 @@ public class CheckpointUtils {
 
             Bukkit.shutdown();
 
-            if(message.contains("Can't dump ghost file")) // File/Jar has been updated
+            if(!message.contains("Can't dump ghost file")) // File/Jar has been updated
                 throw new SecurityException(e);
         } finally {
             // Delete checkpoint