Add max_age_expected to model and pep8

- Adds migration to include `max_age_expected` property on buckets (for recency/health status checks)
- Updated field in bucket model

![](http://24.media.tumblr.com/dd550a48e1a855ce165f945a68c7c596/tumblr_mwmpi8YFgD1rkvauro1_500.jpg)