clang-tidy support followup (actually error travis job with warnings/errors)

As mentioned at https://github.com/mapbox/node-cpp-skel/pull/64#discussion_r133848127 and https://github.com/mapbox/node-cpp-skel/pull/64#discussion_r133848275 when I set up the make tidy target I was unable to find a way to have clang-tidy return non-zero on warnings/errors.

What this means is that the clang-tidy job on travis will only fail when clang tidy notices problems AND fixes them. Then it will leave behind a git diff and the presence of that diff will trigger the job to fail.

But because clang-tidy cannot auto-fix all problems their is a large class of warnings/errors that do not fail the script and therefore may be present but not failing the travis job.

The todo here is to figure out if the usage of clang-tidy can be fixed (are we doing something wrong, forgetting some flag?) to make clang-tidy return a non-zero error code when it encounters warnings/errors.

/cc @GretaCB who saw this recently with shaver - locally @GretaCB saw a number of pretty important clang-tidy warnings that were silently passing on travis since they were not causing the job to fail.

Commencez par la make tidy target et le Travis clang-tidy job, puis lisez les deux commentaires liés de la discussion de la pull request concernant la configuration d’origine. Reproduisez un avertissement qui n’est pas corrigé automatiquement et vérifiez le code de sortie de la commande ainsi que le git diff obtenu. Le travail est terminé lorsque les avertissements ou les erreurs de clang-tidy font échouer de manière fiable le Travis job, y compris pour les problèmes qu’il ne peut pas corriger automatiquement.