Added a configuration option that causes buildout to error if a
  version is picked. This is a nice safety belt when fixing all
  versions is intented, especially whan creating releases.

(This already-desired feature made it easier to write the test for
  the bug fix below. :)

Bug Fixed:

When installing from source releases, a version specification (via a
  buildout versions section) for setuptools was ignored when deciding
  which setuptools to use to build an egg from the source release.