Relax mail gem constraint from ~> 2.5.4 to ~> 2.5, >= 2.5.4

This allows Rails users to install mail 2.6 which relaxes the mime-types
dependency, which is a big win for a lot of people.

Previously, the mail gem restricted mime-types to ~> 1.16 but now it has
expanded to [">= 1.16", "< 3"]

And the mime-types maintainer will also be checking that 2.x releases don't
break mail.

See https://github.com/mikel/mail/pull/713 
https://rubygems.org/gems/mail/versions/2.6.0

Related to https://github.com/rails/rails/pull/15493