Use inheritance_diagram.py provided by Sphinx (≥ 0.6)

Current version of `inheritance_diagram.py` [doesn't work](http://bugs.debian.org/706533) with Sphinx 1.2:

    Extension error:
    Could not import extension inheritance_diagram (exception: cannot import name xfileref_role)

However, Sphinx ≥ 0.6 provides it's own [version](https://bitbucket.org/birkenfeld/sphinx/src/tip/sphinx/ext/inheritance_diagram.py) of `inheritance_diagram` extension which of course works.

A similar changes have already been applied in [nipy](https://github.com/nipy/nipy/commit/3f961418f96c666f) and [nipype](https://github.com/nipy/nipype/commit/b107f0ab87fac9f8).