BUILD: travis-ci: enable regular s390x builds

Previous patch didn't only disable removal of the reg-test but
disabled s390x entirely. Now we enable it like other platforms.
This is an attempt at fixing build issue #504.
diff --git a/.travis.yml b/.travis.yml
index fd136c9..7bcaf0e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -48,6 +48,11 @@
     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
+  - 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"