fixing package docker-engine is not available.

.travis.yml

@@ -45,6 +45,7 @@ before_install:
   - 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
+  - docker version
 
 script: ./travis-build.sh