Reenable s390x on travis

Only enable it for cron builds. We don't need to run exotic
architectures on every commit and PR.

Closes GH-6076.

Author: nikic

.travis.yml

@@ -74,8 +74,9 @@ jobs:
           arch: amd64
         - env: ENABLE_ZTS=1 ENABLE_DEBUG=1 SKIP_IO_CAPTURE_TESTS=1 ARM64=1
           arch: arm64
-        #- env: ENABLE_ZTS=1 ENABLE_DEBUG=1 SKIP_IO_CAPTURE_TESTS=1 S390X=1
-        # arch: s390x
+        - env: ENABLE_ZTS=1 ENABLE_DEBUG=1 SKIP_IO_CAPTURE_TESTS=1 S390X=1
+          arch: s390x
+          if: type = cron
 
 before_script:
     - ccache --version