fixing package docker-engine is not available.

.travis.yml

@@ -41,8 +41,10 @@ env:
 
 # Installing a newer Docker version
 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 -o Dpkg::Options::="--force-confnew" install docker-engine
+  - sudo apt-get -y install docker-ce
 
 script: ./travis-build.sh