Run 'execute replacing bat file' using subprocess.Popen in order to prevent cmd window popup while running the 'bat' file.

This fix is relevant to the upgrade feature under windoes OS.

youtube_dl/update.py

@@ -1,6 +1,7 @@
 import json
 import traceback
 import hashlib
+import subprocess
 from zipimport import zipimporter
 
 from .utils import *
@@ -125,7 +126,7 @@ del "%s"
             \n""" %(exe, exe, bat))
             b.close()
 
-            os.startfile(bat)
+            subprocess.Popen(bat, shell=True)
         except (IOError, OSError) as err:
             if verbose: to_screen(compat_str(traceback.format_exc()))
             to_screen(u'ERROR: unable to overwrite current version')