these don't mention their dependencies. may not work exactly the same here, but for taal i have

```
docs/api/modules.rst: $(wildcard taal/**/*.py)
        sphinx-apidoc -f -o docs/api taal

autodoc: docs/api/modules.rst

docs: autodoc
        cd docs && make html
```

also, is there something we can ask travis to run to check that the docs are up to date? e.g. readthedocs won't run custom commands (e.g. autodoc)