Users can like and unlike a listing

[trello](https://trello.com/c/rIV7ViZ0/19-as-a-user-i-can-like-a-listing)

![image](https://cloud.githubusercontent.com/assets/8655639/5014178/9f582108-6a61-11e4-9b09-f40ba73425b1.png)

Don't use `redirect_to :back`. Where are you actually redirecting to? Try to use a path helper.

I don't like `:back` because it means I have to read more code to know where this is redirecting to. If there's a path helper, I only have to read this one line.