google-chrome-stable の GPG エラーを改善

[nanasess]

Travis CI で以下のようなエラーになってしまう

WARNING: The following packages cannot be authenticated!
 google-chrome-stable
E: There were unauthenticated packages and -y was used without --allow-unauthenticated
apt-get.diagnostics
apt-get install failed
The command "sudo -E apt-get -yq --no-install-suggests --no-install-recommends $(travis_apt_get_options) install dpkg debian-archive-keyring google-chrome-stable" failed and exited with 100 during

• xenial ではデフォルトの google-chrome-stable を使用する
• trusty では --allow-unauthenticated を付与してインストールする