Add missing `getHMACFunc` import in bridgedb.hashring module.
Change an assert to raise an error instead in b.hashring.Hashring.

Asserts are disabled when Python is called with `-OO`. We should avoid
using them in production code.

 * CHANGE an `assert` statement to an explicit test which raises a
   `ValueError` upon failure, in `bridgedb.hashring.Hashring`.
Add missing hashring import to bridgedb.distribute module.

 * ADD `from bridgedb import hashring` to `bridgedb.distribute`.