Fix stalling tests on init systems.

.travis.yml

@@ -2,6 +2,10 @@
 sudo: required
 
 env:
+  global:
+    # https://github.com/travis-ci/travis-ci/issues/6461#issuecomment-239577306
+    DOCKER_VERSION: "1.9.1-0~trusty"
+  matrix:
     - distro: centos7
       init: /usr/lib/systemd/systemd
       run_opts: "--privileged --volume=/sys/fs/cgroup:/sys/fs/cgroup:ro"
@@ -19,7 +23,12 @@ services:
   - docker
 
 before_install:
-  # Pull container
+  # Downgrade to older version of Docker engine.
+  - sudo apt-get update
+  - sudo apt-get remove docker-engine -yq
+  - sudo apt-get install docker-engine=$DOCKER_VERSION -yq --no-install-suggests --no-install-recommends --force-yes -o Dpkg::Options::="--force-confnew"
+
+  # Pull container.
   - 'sudo docker pull geerlingguy/docker-${distro}-ansible:latest'
 
 script: