Ruby

Refactor .travis.yml by introducing travis_retry.sh

Not using official travis_retry.bash, because it's not supporting to
modify backoff seconds.
https://github.com/travis-ci/travis-build/blob/0ad8f1886b2c31994d847e126dc5842b7b3513e3/lib/travis/build/bash/travis_retry.bash

Not using official travis_apt_get_update function because it does not
propagate exit status to be used by retries.
https://github.com/travis-ci/travis-build/blob/0ad8f1886b2c31994d847e126dc5842b7b3513e3/lib/travis/build/bash/travis_apt_get_update.bash

Co-Authored-By: Nobuyoshi Nakada nobu@ruby-lang.org