.travis.yml: Upgrade the used Ubuntu s390x to 22.04 (Jammy).

Upgrade the used Ubuntu version from 20.04 (Focal) to 22.04 (Jammy), alignin
with RubyCI "s390x (Ubuntu)" server.
https://rubyci.org/

Note Travis CI supports Ubuntu 22.04 (Jammy).
https://docs.travis-ci.com/user/reference/jammy/

Set `DFLTCC=0` environment variable as a workaround to avoid the test failures
related to zlib in the `make test-all` and `make test-spec`. The failures can
happen with the zlib library applying the patch madler/zlib#410 to enable the
deflate algorithm producing a different compressed byte stream.

.travis.yml

@@ -87,7 +87,13 @@ env:
   - &s390x-linux
     name: s390x-linux
     arch: s390x
+    dist: jammy
     compiler: gcc
+    env:
+      # Avoid possible test failures with the zlib applying the following patch
+      # on s390x CPU architecture.
+      # https://github.com/madler/zlib/pull/410
+      - DFLTCC=0
 
   - &arm32-linux
     name: arm32-linux