Require _our_ resource_override.

Basically, when anyone else made a 'resource_override' generator,
it'd only require once. Because that's how require works. So let's make
sure that we require ours and nobody else's.

Fixes #251.

lib/active_model_serializers.rb

@@ -15,7 +15,7 @@ class Railtie < Rails::Railtie
 
         Rails::Generators.configure!(app.config.generators)
         Rails::Generators.hidden_namespaces.uniq!
-        require "generators/resource_override"
+        require_relative "generators/resource_override"
       end
 
       initializer "include_routes.active_model_serializer" do |app|