Use `make html` to build the docs under tox

tox.ini

@@ -29,6 +29,7 @@ deps =
 deps =
     Sphinx
     readme
+whitelist_externals = make
 commands =
-    sphinx-build -W -b html -d {envtmpdir}/doctrees docs docs/_build/html
+    make -C docs html BUILDDIR={envtmpdir} "SPHINXOPTS=-W -E"
     python setup.py check -m -r -s