Fix test_image_overlay for py27

Author: Martin Journois

.travis.yml

@@ -17,8 +17,8 @@ before_install:
     - travis_retry conda create --yes -n test python=$PYTHON --file requirements.txt
     - source activate test
     - if [[ "$PYTHON" != "3.3" ]]; then
-        conda install --yes --file requirements-dev.txt
-        conda install -c ioos geopandas
+        conda install --yes --file requirements-dev.txt;
+        conda install -c ioos geopandas;
       fi
     - if [[ "$PYTHON" != "3.5" ]]; then
         travis_retry conda install --yes mock ;