Bump mail gem constraint from [~> 2.5, >= 2.5.4] to ~> 2.6
Bump mail gem constraint from [~> 2.5, >= 2.5.4] to ~> 2.6

Mails downstream dependency (Mime-types) has been shown to decrease
memory usage significantly in its 3.0 release. This memory decrease
will be a big win for users upgrading to Rails 5.

Lets nudge users to upgrade Mail alongside Rails.

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