Add dmarc policy to SES event

[jmoney]

The SES Receipt is missing the attribute for dmarcPolicy of the inbound email.

[codecov-io]

Codecov Report

Merging #102 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #102   +/-   ##
=======================================
  Coverage   71.34%   71.34%           
=======================================
  Files          17       17           
  Lines         698      698           
=======================================
  Hits          498      498           
  Misses        156      156           
  Partials       44       44

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[bmoffatt]

aligns with https://docs.aws.amazon.com/ses/latest/DeveloperGuide/receiving-email-notifications-contents.html#receiving-email-notifications-contents-receipt-object

Thanks!