Fixed gemspec files attribute

After moving `tasks/migrate.rb` to `lib/tasks/migrage.rb` we should update gemspec `files` attribute accordingly, otherwise bundler will warn during `bundle` commang
```bash
annotate at /bundler/gems/annotate_models-6fe1affeeecf did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  ["tasks/migrate.rake"] are not files
```