fix build for client-only platforms

    make[1]: Entering directory `/builddir/build/BUILD/beaker-0.12.0/documentation'
    python -c '__requires__ = ["CherryPy < 3.0", "Sphinx >= 1.0"]; import pkg_resources; execfile("/usr/bin/sphinx-build")' -b man -d _build/doctrees   . _build/man
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/usr/lib/python2.7/site-packages/pkg_resources.py", line 2815, in <module>
        working_set.require(__requires__)
      File "/usr/lib/python2.7/site-packages/pkg_resources.py", line 690, in require
        needed = self.resolve(parse_requirements(requirements))
      File "/usr/lib/python2.7/site-packages/pkg_resources.py", line 588, in resolve
        raise DistributionNotFound(req)
    pkg_resources.DistributionNotFound: CherryPy<3.0
docs: mention beaker-init in upgrade instructions