Update databases/py-peewee to 3.1.0.

Backwards-incompatible changes
- Database.bind() has been renamed to Database.bind_ctx(), to more closely
  match the semantics of the corresponding model methods, Model.bind() and
  Model.bind_ctx(). The new Database.bind() method is a one-time operation
  that binds the given models to the database. See documentation:

Other changes
- Removed Python 2.6 support code from a few places.
- Fixed example analytics app code to ensure hstore extension is registered.
- Small efficiency improvement to bloom filter.
- Removed "attention!" from README.

(fhajny)