Set TRAVIS_CMD when no script is given for Python

Fixes https://github.com/travis-ci/travis-ci/issues/2870

Without this, `$TRAVIS_CMD` is set to whatever was executed beforehand
(e.g., `install` or `before_install` command), which could very well
have succeeded, but the absence of `script` will display the confusing
message:

    The command "$TRAVIS_CMD" exited with 1.