Use the latest version of seed-fu

seed-fu#54 has been merged, so we don't have to use the custom fork anymore.
It also include some changes that are required for Rails 4.2+, at least for now.

See https://github.com/rails/rails/commit/d6c1205584b1ba597db4071b168681678b1e9875#commitcomment-7502487
Merge pull request #2690 from chancancode/use_latest_seed_fu

Use the latest version of seed-fu