Update to latest Sphinx

[gtback]

I tried updating to Sphinx 1.5.5, but our autoentity extension doesn't seem to be working with it, so I rolled back to 1.3.6 (the latest 1.3.X). Sphinx 1.5.5 also doesn't support Python 2.6 or 3.3.

Another error received is:

/Users/gback/code/python-cybox/cybox/utils/autoentity.py:2: DeprecationWarning: sphinx.util.pycompat.class_types is deprecated and will be removed in Sphinx 1.4, please use the standard library version instead.
  from sphinx.util.pycompat import class_types

[emmanvg]

Considering that 2.6 and 3.3 are not supported. Do we actually need to update Sphinx? Is there a particular feature we can leverage not present in 1.3.6?

With the current Sphinx version that we are running (1.3.6) I don't see any issues we need to take care of.

[gtback]

My thought was that we want to use something closer to what ReadTheDocs uses by default. I don't think this is an urgent issue, since we install and use our own Sphinx on ReadTheDocs anyway (I think), so I'll remove it from this milestone. But ideally we would get to it eventually.