Remove .travis specific docker version constraint

.travis.yml

@@ -2,9 +2,6 @@
 sudo: required
 
 env:
-  global:
-    # https://github.com/travis-ci/travis-ci/issues/6461#issuecomment-239577306
-    DOCKER_VERSION: "1.9.1-0~trusty"
   matrix:
     - distro: ernestasposkus/centos7
       init: /usr/lib/systemd/systemd
@@ -17,11 +14,6 @@ services:
   - docker
 
 before_install:
-  # Downgrade to specific version of Docker engine.
-  - sudo apt-get update
-  - sudo apt-get remove docker-engine -yq
-  - sudo apt-get install docker-engine=$DOCKER_VERSION -yq --no-install-suggests --no-install-recommends --force-yes -o Dpkg::Options::="--force-confnew"
-
   # Pull container.
   - 'sudo docker pull ${distro}:latest'