Revert ".travis.yml: Disable allow_failures for arm32."

[junaruga]

This reverts commit 49d4421.

Unfortunately, Travis arm64 pipeline is unstable.
https://travis-ci.community/t/14028

[junaruga]

Hmm, I cannot open even the page below properly.
https://app.travis-ci.com/

[junaruga]

The arm64 (arm32) case passed even when the
https://app.travis-ci.com/github/ruby/ruby/jobs/612381722

Worker information
sudo: unable to resolve host travis-job-ruby-ruby-612381722: Name or service not known
sudo: unable to resolve host travis-job-ruby-ruby-612381722: Name or service not known

[junaruga]

It seems that the issues happened in Travis. It's nice to see their public announcements.

https://www.traviscistatus.com/

• Monitoring - Issue found and fixed. Monitoring system.
  Oct 28, 2023 - 13:00 UTC
• Investigating - Travis API errors discovered. Builds may be breaking.
  Oct 28, 2023 - 12:15 UTC

[junaruga]

If Travis announces the issue report publicly like this, we can still keep disabling allow_failures.