Correct height for the checkbox widget

Minor css fix. The checkbox bottom margin was not sufficient. 

(The correct value comes from bootstrap' s `.checkbox` class which cannot be applied here. )

Before:
![bad](https://cloud.githubusercontent.com/assets/2397974/5033453/e6a9004a-6b33-11e4-9c07-b4c242446f72.png)

After:
![better](https://cloud.githubusercontent.com/assets/2397974/5033455/eaa48552-6b33-11e4-8b89-d831201456ab.png)