workaround for sphinx/docutils issue

https://github.com/sphinx-doc/sphinx/issues/3212

.travis.yml

@@ -39,7 +39,9 @@ matrix:
   - os: linux
   - os: linux
     language: docs
     language: docs
     env: DOCS STYLE LINT
     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:
     script:
     - make -C docs html SPHINX_OPTIONS=-W
     - make -C docs html SPHINX_OPTIONS=-W
     - tools/check-style.sh
     - tools/check-style.sh