Have CI test nbextension validation

Author: vidartf

.travis.yml

@@ -36,6 +36,8 @@ script:
   - pip uninstall -y ipywidgettestwidgets
   - pip install .
   - jupyter nbextension enable --py --sys-prefix ipywidgettestwidgets
+  # Validate nbextension (enable does not use exit code):
+  - python -c "from notebook.nbextensions import validate_nbextension; import sys; sys.exit(validate_nbextension('ipywidgettestwidgets/extension') or 0)"
   - pip install jupyterlab
   # Make sure our lab extension was installed.
   - jupyter labextension list