tag がねーよってエラーでてレポート作成に失敗しちょる

```
/home/gongo/test/vendor/bundle/ruby/1.9.1/gems/turnip_formatter-0.2.4/lib/turnip_formatter/scenario.rb:70:in `tags': undefined method `[]' for nil:NilClass (NoMethodError)
	from /home/gongo/test/vendor/bundle/ruby/1.9.1/gems/turnip_formatter-0.2.4/lib/turnip_formatter/printer/tab_tag_statistics.rb:60:in `block in group_by_tag'
	from /home/gongo/test/vendor/bundle/ruby/1.9.1/gems/turnip_formatter-0.2.4/lib/turnip_formatter/printer/tab_tag_statistics.rb:59:in `map'
	from /home/gongo/test/vendor/bundle/ruby/1.9.1/gems/turnip_formatter-0.2.4/lib/turnip_formatter/printer/tab_tag_statistics.rb:59:in `group_by_tag'
	from /home/gongo/test/vendor/bundle/ruby/1.9.1/gems/turnip_formatter-0.2.4/lib/turnip_formatter/printer/tab_tag_statistics.rb:13:in `print_out'
	from (erb):52:in `template_params_binding'
	from /home/admin/.rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/erb.rb:838:in `eval'
	from /home/admin/.rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/erb.rb:838:in `result'
	from /home/gongo/test/vendor/bundle/ruby/1.9.1/gems/turnip_formatter-0.2.4/lib/turnip_formatter/printer.rb:11:in `render_template'
	from /home/gongo/test/vendor/bundle/ruby/1.9.1/gems/turnip_formatter-0.2.4/lib/turnip_formatter/printer/index.rb:17:in `print_out'
	from /home/gongo/test/vendor/bundle/ruby/1.9.1/gems/turnip_formatter-0.2.4/lib/rspec/core/formatters/turnip_formatter.rb:26:in `dump_summary'
```

このタグっていうのは [lib/turnip_formatter/ext/turnip/rspec.rb](https://github.com/gongo/turnip_formatter/blob/v0.2.4/lib/turnip_formatter/ext/turnip/rspec.rb#L50) で定義されており、tags が nil になるタイミングが今いちわかってない。

が、まあとりあえずエラー出て数十分のテスト結果が一個も出なくなるのは悲しいので、とりあえず nil だったら `[]` 返すぐらいの気持ちでいこうと思う