Allow failures in Travis ppc64le and s390x
Those jobs are way too unstable and provide very little benefit.
e.g. https://app.travis-ci.com/github/ruby/ruby/jobs/624611955 https://app.travis-ci.com/github/ruby/ruby/jobs/625551620
Allow failures in Travis ppc64le and s390x
Those jobs are way too unstable and provide very little benefit.
e.g.
https://app.travis-ci.com/github/ruby/ruby/jobs/624611955
https://app.travis-ci.com/github/ruby/ruby/jobs/625551620