Fix broken tox docs target

This change prevents me from getting the following errors:

    $ tox -e docs
    GLOB sdist-make: /Users/marca/dev/git-repos/honcho/setup.py
    docs inst-nodeps: /Users/marca/dev/git-repos/honcho/.tox/dist/honcho-0.5.0.zip
    docs runtests: PYTHONHASHSEED='3860990299'
    docs runtests: commands[0] | sphinx-build -W -b html -d /Users/marca/dev/git-repos/honcho/.tox/docs/tmp/doctrees .  /Users/marca/dev/git-repos/honcho/.tox/docs/tmp/html
    Running Sphinx v1.3b1
    making output directory...

    Exception occurred:
      File "conf.py", line 51, in <module>
        from ..honcho import __version__
    KeyError: "'__name__' not in globals"
    The full traceback has been saved in /var/folders/gw/w0clrs515zx9x_55zgtpv4mm0000gp/T/sphinx-err-bu6qxnzx.log, if you want to report the issue to the developers.
    Please also report this if it was a user error, so that a better error message can be provided next time.
    A bug report can be filed in the tracker at <https://bitbucket.org/birkenfeld/sphinx/issues/>. Thanks!
    ERROR: InvocationError: '/Users/marca/dev/git-repos/honcho/.tox/docs/bin/sphinx-build -W -b html -d /Users/marca/dev/git-repos/honcho/.tox/docs/tmp/doctrees .  /Users/marca/dev/git-repos/honcho/.tox/docs/tmp/html'

and:

    Warning, treated as error:
    /Users/marca/dev/git-repos/honcho/doc/api.rst:9: WARNING: autodoc: failed to import module 'honcho.procfile'; the following exception was raised:
    Traceback (most recent call last):
      File "/Users/marca/dev/git-repos/honcho/.tox/docs/lib/python3.4/site-packages/sphinx/ext/autodoc.py", line 378, in import_object
        __import__(self.modname)
    ImportError: No module named 'honcho.procfile'