update Docker before installing

It seems that Docker on Travis messes with the file permission settings
leading to errors such as this:
  warning: directory permissions differ on /usr/

Indeed, the permissions inside the containers are off:
  drwxrwxr-x  1 root root  4096 Jan 23 23:14 usr

I cannot reproduce this locally with Docker 18.09.1-ce. Try upgrading
Docker before installing.

.travis.yml

@@ -8,6 +8,9 @@ services:
 env:
   - BUILD_ARCH=arm32v7
   - BUILD_ARCH=arm64v8
+before_install:
+  - sudo apt update -y && sudo apt install --only-upgrade docker-ce -y
+  - docker info
 install:
   - 'echo ''{ "experimental": true }'' | sudo tee /etc/docker/daemon.json'
   - sudo systemctl restart docker