Update mail dependency
Update mail dependency

* V 2.5.4 raises security advisory:
  Title: Mail Gem for Ruby vulnerable to SMTP Injection via recipient email addresses
  Solution: upgrade to >= 2.6.0
* others have implemented this in forks
* eg https://github.com/anjinfan/savon-multipart/commit/d8ee7bb724133a2819b3a25c5c3ce71e65d41f82

savonrb/savon-multipart