Update Social Links plugin to handle 4.0 Customizer "active" state better

...nt display across themes.

Test site: http://themetester.wordpress.net/2010/03/post-format-link/

Before: 
![1 before-large-text](https://cloud.githubusercontent.com/assets/66797/2769006/1a17f2c6-ca4c-11e3-84cc-d8e45a0e0d11.png)
After: 
![2 after-normal-text](https://cloud.githubusercontent.com/assets/66797/2769010/1f6261f8-ca4c-11e3-96ec-a3313ac7366e.png)