Fail to build with Sphinx > 1.6.0

[jd]

Commit e8e63b1e9968abbaff46ddd7917d2b53cb505276 in Sphinx removed sphinx.builders.html.StandaloneHTMLBuilder.default_sidebars so build now fails with:

Exception occurred:
  File "/Users/jd/Source/gnocchi/.tox/docs-gnocchi.xyz/lib/python2.7/site-packages/sphinxcontrib/versioning/sphinx_.py", line 60, in builder_inited
    app.config.html_sidebars['**'] = StandaloneHTMLBuilder.default_sidebars + ['versions.html']
AttributeError: type object 'StandaloneHTMLBuilder' has no attribute 'default_sidebars'

[paddy-hack]

This can be fixed by applying #46 or #50