Enablie IPv6 on Travis s390s case again.

It seems the issue was fixed by Travis.
See https://travis-ci.community/t/6719/5 .

.travis.yml

@@ -133,8 +133,6 @@ env:
     name: s390x-linux
     arch: s390x
     <<: *gcc-8
-    env:
-      - BEFORE_INSTALL="tool/travis_disable_ipv6.sh"
 
   - &jemalloc
     name: --with-jemalloc

tool/travis_disable_ipv6.sh

@@ -1,11 +0,0 @@
-#!/bin/bash
-set -ex
-ip a
-sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1
-sudo sysctl -w net.ipv6.conf.default.disable_ipv6=1
-sudo sysctl -w net.ipv6.conf.lo.disable_ipv6=1
-ip a
-
-cat /etc/hosts
-sudo ruby -e "hosts = File.read('/etc/hosts').sub(/^::1\s*localhost.*$/, ''); File.write('/etc/hosts', hosts)"
-cat /etc/hosts