Add ViewPatterns extension to Cabal file

If I understand it right, Yesod 1.4 [adds a dependency on the ViewPatterns extension](http://www.yesodweb.com/blog/2014/09/announcing-yesod-1-4). Without this extension, for a route like `/articles/#Int ArticleR GET` you'll get this error:

```
Foundation.hs:48:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_alQq
    Use ViewPatterns to enable view patterns
```

So it seems right to add that extension to the scaffolding.