Remove plug_graphql as relay includes it

Author: Josh Price

README.md

@@ -23,10 +23,7 @@ It's important that you understand GraphQL first and then Relay second. Relay is
   1. Add graphql_relay to your list of dependencies in `mix.exs`:
 
         def deps do
-          [
-            {:graphql_relay, "~> 0.0.16"},
-            {:plug_graphql, "~> 0.2"} # Most likely going to need this as well
-          ]
+          [{:graphql_relay, "~> 0.0.16"}]
         end
 
 ## Configuration