don't fail if docker server is not running (#422) (#423)

Author: vchepkov

lib/facter/docker.rb

@@ -51,7 +51,7 @@ def interfaces
 Facter.add(:docker_server_version) do
   setcode do
     docker_version = Facter.value(:docker_version)
-    if docker_version && docker_version['Server'].is_a?(Hash)
+    if docker_version && !docker_version['Server'].nil? && docker_version['Server'].is_a?(Hash)
       docker_version['Server']['Version']
     else
       nil