fix a font issue with qt4.x under yosemite

Previous:
![screen shot 2014-11-28 at 16 15 28](https://cloud.githubusercontent.com/assets/2123461/5225085/03d91974-771a-11e4-8ca0-d0b1c5518a9e.png)

Now:
![screen shot 2014-11-28 at 16 15 05](https://cloud.githubusercontent.com/assets/2123461/5225086/0df8fb36-771a-11e4-9fa8-507bf9afe548.png)

More information is related to [QTBUG-40833](https://bugreports.qt-project.org/browse/QTBUG-40833)