workaround for sphinx/docutils issue

https://github.com/sphinx-doc/sphinx/issues/3212
diff --git a/.travis.yml b/.travis.yml
index cd18e1f..39b4362 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -39,7 +39,9 @@
   - os: linux
     language: docs
     env: DOCS STYLE LINT
-    install: pip install --upgrade sphinx sphinx_rtd_theme flake8 pep8-naming docutils
+    install:
+    - pip install --upgrade sphinx sphinx_rtd_theme flake8 pep8-naming
+    - pip install docutils==0.12
     script:
     - make -C docs html SPHINX_OPTIONS=-W
     - tools/check-style.sh