tools: make Travis commit linting more robust

Use $TRAVIS_COMMIT_RANGE in .travis.yml to avoid merge commits in some
situations.

Refs: https://github.com/nodejs/node/pull/23307#issuecomment-428435859

PR-URL: https://github.com/nodejs/node/pull/23397


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>