Don't run `FactoryGirl#find_definitions` in setup

Running `#find_definitions` in a rake task that is invoked by `./bin/setup`
loads factories twice. This causes `./bin/setup` to fail due to a
`FactoryGirl::DuplicateDefinitionError` if there are pre-existing factories.

This PR also includes a test to ensure `./bin/setup` completes successfully.