specification of pythonocc version

Author: mtezzele

.travis.yml

@@ -48,7 +48,7 @@ install:
       fi
     - source activate test
     - conda install --yes numpy scipy matplotlib pip nose vtk sip=4.18
-    - conda install --yes -c https://conda.anaconda.org/dlr-sc pythonocc-core
+    - conda install --yes -c https://conda.anaconda.org/dlr-sc pythonocc-core==0.17
     - pip install setuptools
     - pip install enum34
     - pip install numpy-stl