Fix logger in Downloader being null (#33)

`LogManager#getLogger` only retrieves existing loggers and doesn't create them.

src/main/java/net/minecraftforge/installer/Downloader.java

@@ -15,6 +15,10 @@
  */
 package net.minecraftforge.installer;
 
+import net.minecraftforge.installer.actions.ProgressCallback;
+import org.jetbrains.annotations.CheckReturnValue;
+import org.jetbrains.annotations.Nullable;
+
 import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
@@ -28,15 +32,11 @@
 import java.nio.file.StandardCopyOption;
 import java.util.Objects;
 import java.util.logging.Level;
-import java.util.logging.LogManager;
 import java.util.logging.Logger;
-import net.minecraftforge.installer.actions.ProgressCallback;
-import org.jetbrains.annotations.CheckReturnValue;
-import org.jetbrains.annotations.Nullable;
 
 @CheckReturnValue
 public class Downloader {
-    private static final Logger LOGGER = LogManager.getLogManager().getLogger("Downloading");
+    private static final Logger LOGGER = Logger.getLogger("Downloading");
     public static final LocalSource LOCAL = LocalSource.detect();
 
     private LocalSource localSource;