Log Message:
msmtp: Update to 1.8.15

Changelog:
Version 1.8.15:
- Added support for SCRAM-SHA-256 authentication via GNU SASL

Version 1.8.14:
- Added support for libtls as an alternative to GnuTLS

Version 1.8.13:
- Added support for XOAUTH2, the predecessor of OAUTHBEARER.
- The passwordeval command can now handle very long input, which can be
  necessary for OAUTHBEARER and XOAUTH2.
- GnuTLS >= 3.4 is required