Fixed ambiguous layout warning.

I tried to fix this warning.

![screen shot 2014-05-01 at 6 04 33 pm](https://cloud.githubusercontent.com/assets/291175/2850995/aa9b8ee2-d10f-11e3-8cbe-75df643d89b1.png)

I added the source list badge's missing constraint, and lowered the priority of the cell's text field's constraint.