Pin docker version in vagrant VM

Otherwise it will always install the latest docker version, which will be
undesirable when new major releases are done, until they are well tested.

For instance, docker 1.9 was killing my VM when spawning a kubernetes cluster,
due to a high CPU consumption, probably due to
moby/moby#17720

Vagrantfile

@@ -5,7 +5,7 @@ require './vagrant-common.rb'
 vm_ip = "172.16.0.3" # arbitrary private IP
 
 pkgs = %w(
-  docker-engine
+  docker-engine=1.8.3-0~vivid
   aufs-tools
   build-essential
   ethtool