.travis.yml: Disable allow_failures for arm32.

I want to disable allow_failures if possible to avoid unintentional new
failures. It seems that the arm32 case is stable.

Feel free to enable allow_failures again when you see the case is unstable.

.travis.yml

@@ -110,7 +110,7 @@ matrix:
     # - <<: *s390x-linux
   allow_failures:
     # Allow failures for the unstable jobs.
-    - name: arm32-linux
+    # - name: arm32-linux
     # - name: arm64-linux
     - name: ppc64le-linux
     - name: s390x-linux