increase max difference for text box test

The text box test has started failing (intermittently?) with 4722 different pixels, but it looks like the rendering is correct, and it's just a difference in the font that causes the problem (reference image on the left; Travis result on the right):

![textboxtest](https://cloud.githubusercontent.com/assets/305455/5533325/9cd11cd8-89fa-11e4-8704-25dce5788872.png) ![textboxtest-travis](https://cloud.githubusercontent.com/assets/305455/5533331/ca1ca478-89fa-11e4-957c-9bdef067e087.png)

So this branch updates the maximum number of pixels that can be different for the test.