Optimize the roles_name memory allocation using pluck

Avoid to instantiate models and just use the the returned values as they are

lib/rolify/role.rb

@@ -86,7 +86,7 @@ def remove_role(role_name, resource = nil)
     deprecate :has_no_role, :remove_role
 
     def roles_name
-      self.roles.select(:name).map { |r| r.name }
+      self.roles.pluck(:name)
     end
 
     def method_missing(method, *args, &block)