Switch one of Travis CI Ruby builds to use s390x arch

Check if the tests work correctly in big endian environment.

Author: vadz

.travis.yml

@@ -351,6 +351,7 @@ matrix:
       sudo: required
       dist: xenial
     - os: linux
+      arch: s390x
       env: SWIGLANG=ruby CPP11=1
       sudo: required
       dist: xenial