Use config.before_eager_load

We want to make sure the gem is fully loaded in a production context
before the application begins eagerly loading, otherwise we emit a
warning. So, let's use config.before_eager_load to make sure gem eager
loading happens before the app.

lib/graphql/railtie.rb

@@ -10,7 +10,9 @@ module GraphQL
   class Railtie < Rails::Railtie
     config.graphql = ActiveSupport::OrderedOptions.new
     config.graphql.parser_cache = false
-    config.eager_load_namespaces << GraphQL
+    config.before_eager_load do
+      GraphQL.eager_load!
+    end
 
     initializer("graphql.cache") do |app|
       if config.graphql.parser_cache