Make G+ button same size as other buttons

The default G+ button size is slightly larger than the Facebook and Twitter buttons.  Use the "medium" size to make the size even.

I've taken screenshots with the modification- if you check the live site, the G+ button is currently slightly bigger.

![screen shot 2014-01-08 at 2 15 10 am](https://f.cloud.github.com/assets/933422/1867385/f876320e-784d-11e3-92a3-871a0806fc3a.png)
![screen shot 2014-01-08 at 2 14 52 am](https://f.cloud.github.com/assets/933422/1867386/f876589c-784d-11e3-92b3-28ac4bcc28e9.png)