TST: Run the nbsphinx module on Travis-CI (to check Sphinx imports)

Author: mgeier

.travis.yml

@@ -33,6 +33,7 @@ install:
   - pip install pandas
   - pip install sphinxcontrib-bibtex
 script:
+  - python -m nbsphinx
   - python -m sphinx doc/ doc/_build/ -b html
   - python -m sphinx doc/ doc/_build/ -b latex
   - python -m sphinx doc/ doc/_build/ -b epub