Tweaked update_version.py to leave PHP notes blank

Until now update_version.py has copied the notes from the previous PHP
release, which is not very useful.

Author: Adam Cozzette

update_version.py

@@ -317,7 +317,7 @@ def CreateNode(tagname, indent, children):
           FindAndClone(root, 'date'),
           FindAndClone(root, 'time'),
           FindAndClone(root, 'license'),
-          FindAndClone(root, 'notes')
+          CreateNode('notes', 3, []),
         ])
       changelog.appendChild(release)
       changelog.appendChild(document.createTextNode('\n '))