Lock `mail` gem to ~> 2.5.4

Rails 4.1.6+ will relax the mail gem version requirement to `~> 2.5, >= 2.5.4`.
However, mail gem 2.6.x currently does not work with discourse because of the
reference to `Mail::RFC2822Parser` in `lib/email.rb`. This ensure discourse
would continue to work with Rails 4.1.6+ when it is released.
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
Merge pull request #2689 from chancancode/lock_mail_gem

Lock `mail` gem to ~> 2.5.4
comment gemfile