Enable git update alias.

Please refer to:

- [local repository をよいかんじに remote に同期させる git update - Qiita [キータ]](http://qiita.com/tomy_kaira/items/2e1d575202e14a28b600)
- [local repository をよいかんじに remote に同期させる git update - tomykaira makes love with codes](http://tomykaira.hatenablog.com/entry/2013/05/22/001914)

This commit fixes #255.
Merge branch 'id/255'