This is a minor maintenance release.
- Improve trapping of TLS errors during connection so that they don't cause warnings, and are reported better in debug output
- Amend test suite so it uses PHPUnit version 4.8, compatible with older versions of PHP, instead of the version supplied by Travis-CI
- This forces pinning of some dev packages to older releases, but should make travis builds more reliable
- Test suite now runs on HHVM, and thus so should PHPMailer in general
- Improve Czech translations
- Add links to CVE-2017-5223 resources