old_tree_timestamp: fix UnicodeDecodeError (530982)

Use _unicode_decode to safely decode bytes returned from time.strftime
for python-2.x (_unicode_decode allows unicode to pass through for
python-3.x).

X-Gentoo-Bug: 530982
X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=530982
Acked-by: Brian Dolbec <dolsen@gentoo.org>