Travis s390x-linux is too unstable

It has caused errors by ENOSPC like
https://travis-ci.org/github/ruby/ruby/jobs/682520961 too often.

I cleared all cache of Travis yesterday, but it didn't help it. Until
somebody figures out how to fix it, let me exclude it from CI status
reports to avoid false alerts.

.travis.yml

@@ -426,10 +426,7 @@ matrix:
   include:
     # Build every commit:
     - <<: *x86_64-linux
-    - <<: *arm64-linux
     - <<: *i686-linux
-    - <<: *arm32-linux
-    - <<: *s390x-linux
     - <<: *pedanticism
     - <<: *assertions
     - <<: *baseruby
@@ -437,6 +434,9 @@ matrix:
     - <<: *rubyspec27
     - <<: *dependency
     # Build every commit (Allowed Failures):
+    - <<: *arm32-linux
+    - <<: *arm64-linux
+    - <<: *s390x-linux
     - <<: *ASAN
     - <<: *MSAN
     - <<: *UBSAN
@@ -451,10 +451,11 @@ matrix:
     - <<: *CALL_THREADED_CODE
     - <<: *NO_THREADED_CODE
   allow_failures:
-    - name: -fsanitize=address
-    - name: -fsanitize=memory
     - name: arm32-linux
     - name: arm64-linux
+    - name: s390x-linux
+    - name: -fsanitize=address
+    - name: -fsanitize=memory
   fast_finish: true
 
 before_script: