docker should already be installed by services

This package no longer exists so trying to install it broke our build.
It has been renamed docker-ce.

.travis.yml

@@ -15,7 +15,7 @@ branches:
 
 before_install:
     - sudo apt-get update
-    - sudo apt-get install -y docker-engine liblxc1 lxc-dev lxc
+    - sudo apt-get install -y liblxc1 lxc-dev lxc
     - sudo apt-get install -y qemu
     - ./scripts/install_rkt.sh
     - ./scripts/install_consul.sh