Fix warnings with recent thor versions

Otherwise running things like `bin/rails g --help` with `draper`
installed leads to warnings like this:

```
Expected string default value for '--decorator'; got true (boolean)
```

See https://github.com/erikhuda/thor/issues/538 for reference.

lib/generators/controller_override.rb

@@ -5,13 +5,13 @@ require "rails/generators/rails/scaffold_controller/scaffold_controller_generato
 module Rails
   module Generators
     class ControllerGenerator
-      hook_for :decorator, default: true do |generator|
+      hook_for :decorator, type: :boolean, default: true do |generator|
         invoke generator, [name.singularize]
       end
     end
 
     class ScaffoldControllerGenerator
-      hook_for :decorator, default: true
+      hook_for :decorator, type: :boolean, default: true
     end
   end
 end