try travis with more recent docker version

.travis.yml

@@ -13,6 +13,12 @@ env:
     - IMAGE_NAME=ci/testing
     - DOCKERFILE=docker/Dockerfile.testing
 
+before_install:
+  - curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
+  - sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
+  - sudo apt-get update
+  - sudo apt-get -y install docker-ce
+
 script:
   - docker build -f "$DOCKERFILE" -t "$IMAGE_NAME" .
   - docker run -it --rm "$IMAGE_NAME"