Add nbconvert etc in .travis

Author: Martin Journois

.travis.yml

@@ -16,6 +16,9 @@ before_install:
     - travis_retry conda create --yes -n test python=$PYTHON --file requirements.txt
     - source activate test
     - conda install --yes --file requirements-dev.txt;
+    - if [[ "$PYTHON" == "3.4" ]]; then
+        travis_retry conda install --yes nbconvert jupyter_client ipykernel;
+      fi
     - travis_retry pip install
 
 script: