Log Message:
Update ruby-mail25 to 2.5.5.

== Version 2.5.5 - 2017-06-09 Jeremy Daer <jeremydaer@gmail.com>

Security:
* #1097 – SMTP security: prevent command injection via To/From
   addresses. (jeremy)

Bugs:
* #633 – Cope with message parts that have an empty Content-Type
   (ThomasKoppensteiner, zeepeeare)
* #689 - Fix Exim delivery method broken by #477 in 2.5.4. (jethrogb)