Add comment to .travis.yml

.travis.yml

@@ -13,6 +13,7 @@ install:
   - sudo -H apt-get install -qq -y pyqt4-dev-tools
   - sudo -H apt-get install -qq -y python-qt4
   - sudo -H pip install -U pip setuptools
+  # scikit-image needs numpy and to be installed by pip
   - sudo -H pip install numpy scikit-image
   - sudo -H python setup.py install