Use pip2 rather than pip

Travis OSX only has pip2/pip3, not pip.
cf. travis-ci/travis-ci#8829

Author: daviddrysdale

.travis.yml

@@ -105,7 +105,7 @@ install:
  - gclient sync --disable-syntax-validation
  - popd
  - export PYTHONPATH=$(python -m site --user-site)
- - pip install --user --upgrade -r python/requirements.txt
+ - pip2 install --user --upgrade -r python/requirements.txt
 
 script:
  - getconf _NPROCESSORS_ONLN