Update zc.buildout from 2.1.0 to 2.1.1

This makes sure we're not running into problems when distribute 0.7 (a
transition package towards the new setuptools, which is a merge of the
former distribute and setuptools projects) is released. Once distribute
0.7 is released, buildout 2.2 will be released soon and we need to
rebootstrap. See [0] for more information.

[0] http://mail.python.org/pipermail/distutils-sig/2013-June/021321.html