Fix membership wrapping issue on profile pages

While browsing Gittip, I noticed that membership lists sometime wrap in weird ways:

![screen shot 2014-01-06 at 1 08 20 pm](https://f.cloud.github.com/assets/688886/1853061/06802bd2-7706-11e3-8c2c-3c9071a50f50.png)

This PR clears the CSS float after every third element to prevent this.

Since it appears that our membership lists aren't deterministic (that is, ordered by date or name), I wasn't able to recreate that specific set of communities. However, this illustrates the solution: http://codepen.io/anon/pen/HCehj