Update to a newer version of the Sphinx theme

Update the sphinx-theme-okfn git submodule to a newer commit. It looks
like it accidentally got reverted to an older commit recently. We can't
use the latest version of sphinx-theme-okfn because it breaks a bunch of
stuff, this commit looks like the newest version that 'just works' for
CKAN.
Merge branch 'master' of github.com:okfn/ckan