ARROW-17715: [CI][C++][Python] Temporary allow failures for s390x on travis (apache#14138)

The s390x builds currently time out on Travis-CI while compiling Arrow C++.

I'll create a new ticket to follow this up but on the meantime we probably should allow them to fail.

Authored-by: Raúl Cumplido <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: raulcd

.travis.yml

@@ -161,6 +161,8 @@ jobs:
 
   allow_failures:
     - name: "Java on s390x"
+    - name: "C++ on s390x"
+    - name: "Python on s390x"
 
 before_install:
   - eval "$(python ci/detect-changes.py)"