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.

Related: https://github.com/chancancode/discourse/issues/3