Identify a tweet by the class "twitter-tweet", not "twitter-embed"

Followup changeset for https://github.com/GigaOM/go-contentwidgets/issues/17

![screen shot 2014-11-03 at 6 30 49 pm](https://cloud.githubusercontent.com/assets/430385/4892388/a6286de6-63b1-11e4-9dfc-8a8536ecf109.png)


See: https://github.com/GigaOM/gigaom/issues/5852