Fix travis script checking the changelog (#1313)
Fix travis script checking the changelog (#1313)

* Fix changelog check CI script

There was two problems:
- `$TRAVIS_COMMIT` may not be a valid commit, we should use
  `$TRAVIS_COMMIT_RANGE` instead.
- Github's rate limiting was returning a response we didn't expect
  We now fail in this case instead of success. We use a more focused API
  that may have weaker rate limiting.

* Assume the label is not present if query failed

* Print a message when the changelog is not uptodate
ocaml-ppx/ocamlformat,ocaml-ppx/ocamlformat,ocaml-ppx/ocamlformat