update: support working with old versions of PIL/Pillow

Image.close() was added in Pillow 2.4 or so.

fdroidserver/update.py

@@ -1492,7 +1492,7 @@ def extract_apk_icons(icon_filename, apk, apkzip, repo_dir):
             logging.warning(_("Failed reading {path}: {error}")
                             .format(path=icon_path, error=e))
         finally:
-            if im:
+            if im and hasattr(im, 'close'):
                 im.close()
 
     if apk['icons']: