build: lint commit message in separate Travis job

Move the first commit message linting to a separate Travis job. Run the
script in bash debug mode to capture any issues communicating with the
GitHub API (e.g. network issues, rate limits).

PR-URL: https://github.com/nodejs/node/pull/24254


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>