Fix mixpanel js syntax. Should close auth0/manage#92

This should close auth0/manage#92 when updated and merged dependency.

### Before
![image](https://cloud.githubusercontent.com/assets/367429/5445903/e85b2090-8496-11e4-963d-286151f341c2.png)

### Now
![image](https://cloud.githubusercontent.com/assets/367429/5445906/fd14c356-8496-11e4-8b1b-4c52a3fc551e.png)