Replace nth-child with nth-of-type to correct voting colors on errors

This fixes #418.

![errors](https://cloud.githubusercontent.com/assets/2820802/5061272/f731f51e-6d8d-11e4-942d-a9da674a469e.png)

The problem was that `div:nth-child(2)` selects the second child, but only if it is a `div`. This meant that an additional error moved all indices by one.
`div:nth-of-type(2)` selects the second element of type `div`, which is what we want. This might break if the vote input divs contain more divs, but I don't see a reason why we should do that.