Downgrade python-occ to v0.17

Author: ndem0

.travis.yml

@@ -68,9 +68,9 @@ install:
     - source activate test
     - pip install numpy scipy matplotlib vtk nose
     - if [[ "$TOXENV" == "py27" ]]; then
-        conda install --yes -c conda-forge -c dlr-sc -c pythonocc -c oce pythonocc-core python=2.7;
+        conda install --yes -c conda-forge -c dlr-sc -c pythonocc -c oce pythonocc-core=0.17 python=2.7;
       else
-        conda install --yes -c conda-forge -c dlr-sc -c pythonocc -c oce pythonocc-core python=3.6;
+        conda install --yes -c conda-forge -c dlr-sc -c pythonocc -c oce pythonocc-core=0.17 python=3.6;
       fi
     - pip install setuptools
     - pip install coveralls