travis.yml: add arch s390x target

Signed-off-by: Patrick Steuer <patrick.steuer@de.ibm.com>

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10446)

(cherry picked from commit 985412f8c14853b9936852bc7ef4d9438db27b88)

.travis.yml

@@ -36,6 +36,10 @@ matrix:
           arch: arm64
           compiler: gcc
           env: CONFIG_OPTS="--strict-warnings"
+        - os: linux
+          arch: s390x
+          compiler: gcc
+          env: CONFIG_OPTS="--strict-warnings"
         - os: linux-ppc64le
           sudo: false
           compiler: clang