travis: use pip2 instead of pip

.. since now mac osx image expects pip2 or pip3, and doesn't know pip:

0.01s$ pip install --user cpp-coveralls
/Users/travis/.travis/job_stages: line 57: pip: command not found

Ref: https://github.com/travis-ci/travis-ci/issues/8829

Closes https://github.com/curl/curl/pull/2133