Chartbeat only loads if you are in the control group (10%)

Currently Chartbeat is loading for *everyone*, even if the switch is off. This fixes that problem.

/cc @kenlim 

![tumblr_inline_mxio1gmddb1raprkq](https://f.cloud.github.com/assets/84996/1891396/585201ae-7a3c-11e3-9f48-26a3d0fcb1c9.gif)