Removed CommCare Exchange ratings and reviews

Ratings and reviews are gone from the app details:
![commcare_exchange_ratings_gone_2](https://cloud.githubusercontent.com/assets/708421/4977442/769871ee-68e7-11e4-96d8-0b5f010dca20.png)

Sorting by ratings/popularity is also removed:
![commcare_exchange_ratings_gone_1](https://cloud.githubusercontent.com/assets/708421/4977438/6d6530c6-68e7-11e4-8f86-a974eb40606d.png)