Disable tests on s390x

When tests are run in QEMU the job just times out

Author: Susurrus

.travis.yml

@@ -70,7 +70,7 @@ matrix:
       rust: 1.13.0
     - env: TARGET=powerpc64le-unknown-linux-gnu
       rust: 1.13.0
-    - env: TARGET=s390x-unknown-linux-gnu
+    - env: TARGET=s390x-unknown-linux-gnu DISABLE_TESTS=1
       rust: 1.13.0
     - env: TARGET=x86_64-unknown-linux-gnu
       rust: 1.13.0