Fix travis (#245)

Move to the installed default docker version.

.travis.yml

@@ -1,6 +1,6 @@
 sudo: required
 
-dist: xenial
+dist: trusty
 
 language: bash
 
@@ -8,12 +8,6 @@ services:
   - docker
   
 before_install:
-  - sudo sh -c 'echo "deb https://apt.dockerproject.org/repo ubuntu-xenial main" > /etc/apt/sources.list.d/docker.list'
-  - sudo apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
-  - sudo apt-get clean
-  - sudo apt-get update
-  - sudo apt-key update
-  - sudo apt-get -qqy -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold" install docker-engine=1.11.0-0~xenial
   - sudo rm -f /usr/local/bin/docker-compose
   - curl -L https://github.com/docker/compose/releases/download/1.7.1/docker-compose-`uname -s`-`uname -m` > docker-compose
   - chmod +x docker-compose