Group create and update activities using small profile image instead of medium

When creating and updating a group the activity feed entries show a scaled up small image when the medium one looks much better.

Here's the small images:
![smallprofile](https://cloud.githubusercontent.com/assets/70221/4013426/10221362-2a16-11e4-9810-9432cdd9f9e6.png)

Here's what it looks like on the memberships page where the medium image is used:
![medprofile](https://cloud.githubusercontent.com/assets/70221/4013435/2d7f06f4-2a16-11e4-906b-d7f6875f946f.png)