Do not require Docker 1.9 anymore

This was done when docker_build was not supporting the new OCI format
but it has long be done and prevent switching to the new docker_build
repo.

Change-Id: Iac5c43657ece7af942ed9ac5fd3a1a17debab48a
diff --git a/gce/ubuntu-15-10-docker.sh b/gce/ubuntu-15-10-docker.sh
index 03453bb..d35df1d 100644
--- a/gce/ubuntu-15-10-docker.sh
+++ b/gce/ubuntu-15-10-docker.sh
@@ -9,7 +9,7 @@
 apt-get -y update
 # Installing docker might fails because the docker socket is masked.
 # See https://github.com/docker/docker/issues/22847
-apt-get -y install docker-engine=1.9.* \
+apt-get -y install docker-engine \
   || (systemctl unmask docker.socket; service docker start; apt-get install -f)
 
 # Docker group