Test: travis: build up a test matrix incl. no-compile item; cleanup

• drop chrpath and perl packages as dependencies completely
• restructure for readability, minimize separate invocations
• add no-compile (maintenance only) test item to the matrix that runs xml/regression.sh (without any switch and with -B/-S) once all the RNG files are generated (bypassing full-fledged configuration); it also sanity-checks that RNG files, unchecked so far, validate per public relaxng.rng schema themselves