Fix problem with intersphinx not working

[namurphy]

One thing that came up as a side issue of #89 is that the reStructuredText links to Python objects in other packages (i.e., ~sunpy.map.sources.hinode.XRTMap in the docstring for xrt_teem) isn't working. This may be an issue with how intersphinx is set up, or perhaps an incompatibility between different packages used for the docs (e.g., something like sphinx_rtd_theme vs. a new release of sphinx). I took a quick look at what's in the documentation configuration file for Sphinx (docs/conf.py) and didn't see any issues, so I don't know what's causing it.