Do not pass unicode where byte strings are wanted

Some versions of the tarfile module break when unicode data is
passed as metadata (like the version). See
http://bugs.python.org/issue11638 for details.

This change ensures that version number is always a byte strings
under python 2, where the problem exists.

Change-Id: Ic266cd7cce0394d28794ee61afe269f1296be2a2