Fix base class used by rubocop cop

Fix deprecation warning: Inheriting from 'RuboCop::Cop::Cop' is deprecated. Use 'RuboCop::Cop::Base' instead.
For more information, see https://docs.rubocop.org/rubocop/v1_upgrade_notes.html.

lib/smart_todo_cop.rb

@@ -9,7 +9,7 @@ module SmartTodo
       # This Cop does not run by default. It should be added to the RuboCop host's configuration file.
       #
       # @see https://rubocop.readthedocs.io/en/latest/extensions/#loading-extensions
-      class SmartTodoCop < Base
+      class SmartTodoCop < Rubocop::Cop::Base
         HELP = "For more info please look at https://github.com/Shopify/smart_todo/wiki/Syntax"
         MSG = "Don't write regular TODO comments. Write SmartTodo compatible syntax comments. #{HELP}"