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.

Beginne mit dem make tidy target und dem Travis clang-tidy job und lies anschließend die beiden verlinkten Kommentare in der Pull-Request-Diskussion zum ursprünglichen Setup. Erzeuge eine Warnung, die nicht automatisch behoben wird, und überprüfe den Exit-Status des Befehls sowie den resultierenden git diff. Erledigt bedeutet, dass clang-tidy-Warnungen oder -Fehler den Travis job zuverlässig fehlschlagen lassen, einschließlich Problemen, die nicht automatisch behoben werden können.