chore(sauce): use tunnel-identifier and ready-file only on Travis

When running locally, there's not TRAVIS_JOB_NUMBER env variable defined and it screws
the Sauce Connect (it uses a tunnel with empty name), this makes it work locally without defining
TRAVIS_JOB_NUMBER env variable.

Also, if you run the sauce_connect_setup.sh locally, without having SAUCE_CONNECT_READY_FILE, it
does not pass the `--ready-file` argument to avoid Sauce Connect blowing up.