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

Author: k0kubun

.travis.yml

@@ -114,8 +114,8 @@ matrix:
   allow_failures:
     # Allow failures for the unstable jobs.
     # - name: arm64-linux
-    # - name: ppc64le-linux
-    # - name: s390x-linux
+    - name: ppc64le-linux
+    - name: s390x-linux
     # The 2nd arm64 pipeline may be unstable.
     # - name: arm32-linux
   fast_finish: true