added validation for no negative tickets

![screen shot 2014-11-08 at 12 53 58 pm](https://cloud.githubusercontent.com/assets/7917324/4965481/24d3feda-6786-11e4-92cf-feb61201376a.png)
![screen shot 2014-11-08 at 12 56 24 pm](https://cloud.githubusercontent.com/assets/7917324/4965483/3e33ce50-6786-11e4-92c3-9356ef1bd6ab.png)