Create an index on twitter_followers for batch

@frabcus and I just investigated the source of high I/O load on free, and found
it to be [this query](https://github.com/scraperwiki/twitter-follows-tool/blob/d98bef349ebd3a60b57e31c4207eb148242bedff/twfollow.py#L154). We've tried out adding an index to it, and it causes
an order of magnitude less reads.