Make it harder to misconfigure `phpmailer.smtp-protocol`

Summary: Until we implement an "enum" type for config, make this a bit harder to get wrong. A user entered "TLS", but the correct value is "tls". The documentation is consistent about this, but the behavior is sitll surprsing.

Test Plan: eyeballed it

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7778