Do not show current version button on banner for users in a cohort

This prepares our welcome banner so that the 'User current version' button is replaced with a 'Find out more' button. Otherwise users in the cohort will be tempted to hit the 'back to r2' action, which is really intended for those hit came to us from the 'try beta' r2 action.

![screen shot 2014-11-05 at 15 13 22](https://cloud.githubusercontent.com/assets/4549425/4919945/5aef3af0-64fe-11e4-86bc-2ad38bc49825.png)