Add sphinx theme not added by default anymore by sphinx

.travis.yml

@@ -36,7 +36,7 @@ install:
   - source activate pyenv
   - if [[ $TRAVIS_PYTHON_VERSION == '2.7' ]]; then pip install pydot; else pip install pydot-ng; fi
   - pip install . --no-deps
-  - pip install flake8-future-import nose-parameterized==0.5.0
+  - pip install flake8-future-import nose-parameterized==0.5.0 sphinx_rtd_theme
 
 # command to run tests
 env: