Put docker version in the script

.travis.yml

@@ -9,9 +9,6 @@ language: go
 go:
   - 1.6
 
-env:
-  - DOCKER_VERSION=1.9.1
-
 matrix:
   allow_failures:
     - go: tip

scripts/update_docker.sh

@@ -2,10 +2,12 @@
 
 set -ex
 
+DOCKER_VERSION="1.9.1"
+
 sudo stop docker
 sudo rm -rf /var/lib/docker
 sudo rm -f `which docker`
 sudo apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
 echo "deb https://apt.dockerproject.org/repo ubuntu-trusty main" | sudo tee /etc/apt/sources.list.d/docker.list
 sudo apt-get update
-sudo apt-get install docker-engine=$(DOCKER_VERSION)-0~$(shell lsb_release -cs) -y --force-yes
+sudo apt-get install docker-engine=$DOCKER_VERSION-0~`lsb_release -cs` -y --force-yes