travis: don't fail a build when the s390 job failed

Builds for s390x fail due to a qemu bug.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

.travis.yml

@@ -27,6 +27,7 @@ matrix:
     - env: TR_ARCH=docker-test
     - env: TR_ARCH=fedora-rawhide
     - env: TR_ARCH=fedora-rawhide-aarch64
+    - env: TR_ARCH=s390x
 script:
   - sudo make CCACHE=1 -C scripts/travis $TR_ARCH
 after_success: