Travis Update to Docker 1.13

update_docker.sh

#!/usr/bin/env bash
set -euo pipefail

sudo sh -c 'echo "deb https://apt.dockerproject.org/repo ubuntu-$(lsb_release -cs) main" > /etc/apt/sources.list.d/docker.list'
curl -fsSL https://apt.dockerproject.org/gpg | sudo apt-key add -
sudo apt-key fingerprint 58118E89F3A912897C070ADBF76221572C52609D
sudo apt-get update
sudo apt-get -y install "docker-engine=1.13.1-0~ubuntu-$(lsb_release -cs)"

Note: I still had to list docker under services in .travis.yml, otherwise I was running into issues running the docker cli without sudo.

Also note: This update takes around ~15s in my builds.