Update gems to latest versions.
Remove references in tests to the "type" column in the users table.

The "type" column is used internally by ActiveRecord to implement
single-table inheritance between Artist and User models. Our code should
not depend on it in any way..
Remove an extraneous property from the Bounty model.
Fix the bounty error message helper so it doesn't crash if the controller doesn't have an @bounty variable.
Improve organization in the Bounty controller tests by using sub-tests.
Make "name" a not-null property for Artist models.