au: Fix octal-looking version number handling in memento_updater.sh.

I've been seeing (usually harmless?) errors like:

  memento_updater.sh: line 197: printf: 015900: invalid number

when autoupdating, depending on the time of day when I did the build.

Review URL: http://codereview.chromium.org/465060

git-svn-id: svn://chrome-svn/chromeos/trunk@333 06c00378-0e64-4dae-be16-12b19f9950a1