Correct button alignment of round +/- buttons

This corrects the alignment of the +/- buttons. It does this by increasing the size of the buttons to match the default font sizing in WP 4.0. Not sure if there are impacts on older versions of WP - @benhuson, @JustinSainton ?

Before:
![image](https://cloud.githubusercontent.com/assets/1097338/5114199/8704898e-702e-11e4-921f-982dda6e8a24.png)

After:
![image](https://cloud.githubusercontent.com/assets/1097338/5116063/6e4d30e8-703e-11e4-87e3-15ef3ea9410e.png)