Create .version after all files have been downloaded.

script/update-external-binaries.py

@@ -23,9 +23,6 @@ def main():
   rm_rf('external_binaries')
   safe_mkdir('external_binaries')
 
-  with open(version_file, 'w') as f:
-    f.write(VERSION)
-
   if sys.platform == 'darwin':
     download_and_unzip('Mantle')
     download_and_unzip('ReactiveCocoa')
@@ -34,6 +31,9 @@ def main():
     download_and_unzip('directxsdk')
     download_and_unzip('vs2012_crt')
 
+  with open(version_file, 'w') as f:
+    f.write(VERSION)
+
 
 def is_updated(version_file, version):
   existing_version = ''