.travis.yml: allow arm32-linux to fail

It seems fragile now, seemingly due to environmental issues.  Lets allow
it to fail for a while.  Reported by Jun Agura <jaruga@redhat.com>
[ruby-core:97540]

.travis.yml

@@ -457,6 +457,7 @@ matrix:
   allow_failures:
     - name: -fsanitize=address
     - name: -fsanitize=memory
+    - name: arm32-linux
   fast_finish: true
 
 before_script: