This PR is pointed out from https://bugs.ruby-lang.org/issues/16360#note-16 .

This fixes following error that sometimes happens once in a few times
on Travis s390x environment.

$ tool/travis_retry.sh sudo -E apt-add-repository -y "ppa:ubuntu-toolchain-r/test"
+ sudo -E apt-add-repository -y ppa:ubuntu-toolchain-r/test
Error: retrieving gpg key timed out.

+      - bash -cx "${BEFORE_INSTALL}"

For above line, I put bash -cx because when run ${BEFORE_INSTALL} without bash -cx, the sed command outputs the following error. https://travis-ci.org/junaruga/ruby/jobs/633341625#L210

$ ${BEFORE_INSTALL}
sed: -e expression #1, char 1: unknown command: `''

It's useful to see the actual executed command in the Travis log like this.

$ bash -cx "${BEFORE_INSTALL}"
+sed vm_opts.h -e 's/OPT_SUPPORT_JOKE *0/OPT_SUPPORT_JOKE 1/' -i

I added the script tool/travis_disable_ipv6.sh, as I had no idea about how to add the multiple commands to ${BEFORE_INSTALL}.

The ruby code

+sudo ruby -e "hosts = File.read('/etc/hosts').sub(/^::1\s*localhost.*$/, ''); File.write('/etc/hosts', hosts)"

was used in wercker.yml in the past.

I tested s390x-linux case, and I executed the case 5 times continuously, and all the jobs is succeeded. And I want to watch the case keeping it as allow_failures, on master branch for now, until we can see it will be stable.
I tested SUPPORT_JOKE case too that is using ${BEFORE_INSTALL}.