ci: Fix macOS build using pip2 instead of pip

See travis-ci/travis-ci#8829 (comment)

Author: jcfr

.travis.yml

@@ -37,6 +37,7 @@ cache:
     - $HOME/downloads
 
 before_install:
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then mkdir $HOME/bin; ln -s $(which pip2) $HOME/bin/pip; fi
   - pip install scikit-ci==0.13.0 scikit-ci-addons==0.11.0
   - ci_addons --install ../addons