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.

まず make tidy target と Travis clang-tidy job から始め、その後、元のセットアップに関するリンク先の 2 つの pull request の議論コメントを読んでください。自動修正されない警告を再現し、コマンドの終了ステータスと、その結果生じる git diff を確認してください。完了とは、clang-tidy の警告またはエラーによって Travis job が確実に失敗することを意味し、自動的に修正できない問題も含まれます。