Use latest version of docker

.travis.yml

@@ -23,7 +23,12 @@ cache:
     - $GOCACHE
 
 before_install:
-  - docker --version
+  # Update docker to latest version: https://docs.travis-ci.com/user/docker/#installing-a-newer-docker-version
+  - 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 -o Dpkg::Options::="--force-confnew" install docker-ce
+  # Install golangci-lint
   - curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | bash -s -- -b $GOPATH/bin $GOLANGCI_LINT_VERSION
 
 before_script: