Show warning when enabling case sharing for a user group

Refer to: http://manage.dimagi.com/default.asp?135316#766396

![image](https://cloud.githubusercontent.com/assets/3419385/4377968/e5eb814e-4353-11e4-9573-b0a906b8c149.png)
In this situation the input is a single select which maps to the string 'true' or 'false'.  Is there still a more appropriate way to use .is()?
![image](https://cloud.githubusercontent.com/assets/3419385/4379680/d60b0cb8-4362-11e4-98de-d17c648e8bdf.png)