sauce_labs: use a PTY for the wrapper script on OS X
sauce_labs: use a PTY for the wrapper script on OS X

This makes the commands run like they are run in a terminal (colours,
progress bars, etc.), more like they used to before the workaround with
the wrapper script.

final-ci/travis-worker,ireznice/travis-worker,ireznice/travis-worker,travis-ci/travis-worker,gavioto/travis-worker,travis-ci/travis-worker,final-ci/travis-worker,gavioto/travis-worker