Freeze branca commit nb

Author: Martin Journois

.travis.yml

@@ -16,7 +16,7 @@ before_install:
     - travis_retry conda create --yes -n test python=$PYTHON --file requirements.txt
     - source activate test
     - conda install --yes --file requirements-dev.txt;
-    - pip install git+https://github.com/python-visualization/branca.git
+    - pip install git+https://github.com/python-visualization/branca.git@a27055b3a9bd823a917a9a9847efac8dbd590238
     - if [[ "$PYTHON" == "3.4" ]]; then
         travis_retry conda install --yes nbconvert jupyter_client ipykernel;
         conda install --yes -c ioos geopandas;