Enable docker on Travis

.travis.yml

@@ -1,11 +1,29 @@
 language: python
+sudo: required
+services:
+  - docker
 python:
   - 2.7
   - 3.3
   - 3.4
   - 3.5
   - pypy
 
+env:
+  global:
+    - DOCKER_VERSION=1.10.2-0~trusty
+    - DOCKER_COMPOSE_VERSION=1.6.0
+
+before_install:
+  # upgrade docker-engine to specific version
+  - sudo apt-get -o Dpkg::Options::="--force-confnew" install -y docker-engine=${DOCKER_VERSION}
+
+  # reinstall docker-compose at specific version
+  - sudo rm /usr/local/bin/docker-compose
+  - curl -L https://github.com/docker/compose/releases/download/${DOCKER_COMPOSE_VERSION}/docker-compose-`uname -s`-`uname -m` > docker-compose
+  - chmod +x docker-compose
+  - sudo mv docker-compose /usr/local/bin
+
 install:
   - pip install -e .[test]
   - pip install coverage coveralls