v1 bootstrap doesn't constrain the buildout version for dependent parts

[freddrake]

Parts that depend on zc.buildout can end up with buildout 2 even though bin/buildout is using buildout 1, with the bootstrap script from

http://downloads.buildout.org/1/bootstrap.py

Pinning the version of zc.buildout explicitly allows working around this problem.

To reproduce:

1. Clone the sample project:

   https://github.com/freddrake/bootstrap-v1-bug-demo

2. Bootstrap:

   $ wget -q http://downloads.buildout.org/1/bootstrap.py
   $ python bootstrap.py
   

3. Build and run the generated script::

   $ bin/buildout
   $ bin/myscript
   mydemo 0
   zc.buildout 2.0.1
   setuptools 0.6c12dev-r88846
   wsgiref 0.1.2
   Python 2.6.7