Update ruby-actionmailbox60 to 6.0.3.
## Rails 6.0.3 (May 06, 2020) ##
* Update Mandrill inbound email route to respond appropriately to HEAD requests for URL health checks from Mandrill.
*Bill Cromie*
Add ruby-actionmailbox60 package version 6.0.2.2.
Action Mailbox
Action Mailbox routes incoming emails to controller-like mailboxes for
processing in Rails. It ships with ingresses for Mailgun, Mandrill, Postmark,
and SendGrid. You can also handle inbound mails directly via the built-in
Exim, Postfix, and Qmail ingresses.
The inbound emails are turned into `InboundEmail` records using Active Record
and feature lifecycle tracking, storage of the original email on cloud storage
via Active Storage, and responsible data handling with on-by-default
incineration.
These inbound emails are routed asynchronously using Active Job to one or
several dedicated mailboxes, which are capable of interacting directly with
the rest of your domain model.
You can read more about Action Mailbox in the [Action Mailbox
Basics](https://edgeguides.rubyonrails.org/action_mailbox_basics.html) guide.
This is for Ruby on Rails 6.0.
