Upgrade docker-engine and allow downgrades

.travis.yml

@@ -4,13 +4,13 @@ services:
 
 env:
     global:
-        - DOCKER_VERSION=1.12.1-0~trusty
+        - DOCKER_VERSION=1.12.3-0~trusty
 
 before_install:
   # list docker-engine versions
   - apt-cache madison docker-engine
   # upgrade docker-engine to specific version
-  - sudo apt-get -o Dpkg::Options::="--force-confnew" install -y docker-engine=${DOCKER_VERSION}
+  - sudo apt-get -o Dpkg::Options::="--force-confnew" install -y --allow-downgrades docker-engine=${DOCKER_VERSION}
   - docker version
   - docker info
   - sudo add-apt-repository ppa:duggan/bats --yes