Remove hardcoded dependencies of zc.buildout in bootstrap: easy_install
will anyway install all the actual dependencies of the pinned version
of zc.buildout, and return a working set containing them all.

Before this, when the pinned zc.buildout had other dependencies than
the ones hardcoded, they were installed correctly but bootstrap took
only the hardcoded ones into account and wrongly failed due to the
other dependencies being missing.

This is needed for buildout 3 because the dependencies now include
pip and wheel in addition to setuptools.