Specify the version of sass as 3.2.13.

It seems there have some bugs in version 3.3.x.

I have many warnings when I push the tree to Heroku. Like this.

```
       Compiled jquery.ui.all.css  (193ms)  (pid 2062)
       Warning. Error encountered while saving cache /tmp/build_03075db0-0735-449c-a8e3-712028080bbc/tmp/cache/sass/4976cfac0d7960e5a7c0997bd9977f9be1abb388/main.css.scssc: can't dump anonymous class #<Class:0x00000004107170>
```

I googled and found the reason of this issue is cuaused by bugs on sass 3.3.x.
(See also: http://stackoverflow.com/questions/22276991/heroku-error-encountered-while-saving-cache)