Allow reinvoking seed tasks.

Rake allows a task to be "reenabled" to run more than once in the same overall
rake process. Prior to this commit, if you attempted to re-invoke a seedbank
task, the task expressing its dependencies would be redefined. The effect of
that definition was that the seed data code would effectively run twice on the
second invocation, three times on the third, etc.

Reinvoking seed tasks can be useful within a test suite. It allows you to define
real seed data for just the tests that need it, avoiding overhead and the
complications of a non-empty database for tests that don't need the seed data.