update diffview to latest

Looks slightly better in case of http://ly.g0v.tw/bills/1722L15417 , but not good enough.  I'll try the other diff library when I have a chance.
![diff](https://f.cloud.github.com/assets/82666/1415008/44c5d4da-3ecd-11e3-9cf4-e3f65c744755.png)