tox: update doc build commands

- Run html builder with warnings treated as errors.
- Run the linkcheck builder.

Fixes python-virtualenvwrapper#15

Author: dhellmann

tox.ini

@@ -42,7 +42,9 @@ commands =  ksh ./tests/run_tests {envdir} []
 deps =
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/docs/requirements.txt
-commands = python setup.py build_sphinx
+commands =
+    sphinx-build -W -j auto -b html -d docs/build/doctrees docs/source docs/build/html
+    sphinx-build -W -j auto -b linkcheck -d docs/build/doctrees docs/source docs/build/linkcheck
 
 [testenv:pkglint]
 deps=