use the last zc.buildout version that supports Python 2.6.

slapos/rebootstrap/__init__.py

@@ -92,6 +92,8 @@ Buildout will be restarted automatically to have this change applied.
       # rerun buildout with only neeeded section to reuse buildout
       # ability to calcuate all dependency
       args.extend([
+        # use the last zc.buildout version that supports Python 2.6
+        'versions:zc.buildout=2.5.2+slapos005',
         # chroot to rebootstrap directory
         'buildout:directory=%s' % self.rebootstrap_directory,
         # preserve bin-directory outside the chroot.