FIX: Make Discourse.redis change backwards compatible

25ea0b3a broke docker manager when it was run against earlier versions
of Discourse, this attempts to work around the issue by introducing
the method if it is missing.

plugin.rb

@@ -6,6 +6,16 @@
 # authors: Robin Ward, Sam Saffron
 # url: https://github.com/discourse/docker_manager
 
+
+if !defined? ::Discourse.redis
+  # HACK: make Discourse.redis work on old versions of Discourse
+  module ::Discourse
+    def self.redis
+      $redis # rubocop:disable Style/GlobalVars
+    end
+  end
+end
+
 module ::DockerManager
   # should be automatic, but something is weird
   load File.expand_path(File.dirname(__FILE__)) + '/app/helpers/application_helper.rb'