Fail source check on Travis file validation

[bbody]

Currently travis lint (should be travis lint -x) doesn't return a status code as one of our linting warnings is invalid and thus would break any builds. Once this ticket is fixed https://github.com/travis-ci/travis-yaml/issues/58 then should then travis lint -x. Until then, should be an honour system among developers, as it will still spit out the problems. Travis also will just stop working, so should not be a problem.