fix doc upload

Author: jlwalke2

.travis.yml

@@ -56,7 +56,7 @@ matrix:
 
     - stage: upload documentation
       python: 3.6
-      install: pip install sphinx
+      install: pip install sphinx pyyaml
       script:
       - sphinx-build -Ean -b html -j auto -D todo_include_todos=0 ./doc ./doc/_build/html
       - touch ./doc/_build/html/.nojekyll