Handle RAT 0.13 and 0.17 output (#423)

This updates the RAT job checks to accept the old 0.13 summary
format and the new 0.17 summary format. The selection is based on
whether the checked-out Traffic Server tree includes ci/apache-rat-0.17.jar.

This also passes the RAT output path into the helper explicitly so the
validation logic is clearer while ATS rolls forward to RAT 0.17.