Output available instances and fallback URL to redis

Once a list of available URLs has been determined for a particular
service, the list is written as "service -> [list of instances]" to a
local redis connection. These can then be used in the greater routing
logic to pick a random instance from the list, or use a fallback
instance if none are determined to be available.

mix.exs

@@ -25,7 +25,8 @@ defmodule RouterExample.MixProject do
       {:jason, "~> 1.1"},
       {:plug_cowboy, "~> 2.0"},
       {:poison, "~> 5.0"},
-      {:httpoison, "~> 1.8"}
+      {:httpoison, "~> 1.8"},
+      {:redix, "~> 1.1"}
     ]
   end
 end