Fix docs build.

Building the docs with this patch gives the following output:
```
$ make
../env/bin/swaddle ../tests/env ../env/bin/sphinx-build -b html -d _build/doctrees  . _build/html
Making output directory...
Running Sphinx v1.2b3
loading pickled environment... not yet created
loading intersphinx inventory from http://docs.python.org/objects.inv...
building [html]: targets for 31 source files that are out of date
updating environment: 31 added, 0 changed, 0 removed
reading sources... [100%] index                                                                                                                                                   
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] index                                                                                                                                                    
writing additional files... (24 module code pages) _modules/index
 genindex py-modindex search
copying static files... done
copying extra files... dumping search index... done
dumping object inventory... done
build succeeded.

Build finished. The HTML pages are in _build/html.
```