イベントの「参加を辞める」ボタンが変だったの修正

# before

![51eb8cf05268d3e8aed462d4014ff116](https://cloud.githubusercontent.com/assets/1044064/5297335/16195fb4-7bf1-11e4-91a1-9aa76f3fa1a5.png)


# after 

![63c6a13ed5feded3aff808ea0b3b7f17](https://cloud.githubusercontent.com/assets/1044064/5297329/f9e29eb4-7bf0-11e4-97f1-d4933ca9619d.png)