Fog jg user unique vote

Fixed opinions so that user can only make one per matchup. Also fixed tests!

![husky_puppy](https://cloud.githubusercontent.com/assets/6283512/4446619/159ac99a-4804-11e4-9882-28b0e62e7259.jpg)

Can do this in a single conditional:

```erb
<% if current_user && !current_user.has_opinion?(@matchup.id) %>
  # ...
<% end %>
```
```