Updated Travis script to use latest version of docker

Author: shazChaudhry

.travis.yml

@@ -1,8 +1,11 @@
 sudo: required
 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-ce
-  - docker version
+  - docker --version
+
 services:
   - docker