BUILD: travis-ci: enable s390x builds

reg-tests/seamless-reload/abns_socket.vtc is skipped due to #504
diff --git a/.travis.yml b/.travis.yml
index fd136c9..d263cf7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -48,6 +48,13 @@
     compiler: clang
     env: TARGET=linux-glibc OPENSSL_VERSION=1.1.1d
   - os: linux
+    arch: s390x
+    if: type != cron
+    compiler: gcc
+    env: TARGET=linux-glibc OPENSSL_VERSION=1.1.1d
+    before_script:
+      - rm reg-tests/seamless-reload/abns_socket.vtc # please, see https://github.com/haproxy/haproxy/issues/504
+  - os: linux
     if: type == cron
     compiler: clang
     env: TARGET=linux-glibc OPENSSL_VERSION=1.1.1d COVERITY_SCAN_PROJECT_NAME="Haproxy" COVERITY_SCAN_BRANCH_PATTERN="*" COVERITY_SCAN_NOTIFICATION_EMAIL="chipitsine@gmail.com" COVERITY_SCAN_BUILD_COMMAND="make CC=clang TARGET=$TARGET $FLAGS 51DEGREES_SRC=$FIFTYONEDEGREES_SRC"