Support Swift Mailer 6, drop Swift Mailer 5

• Enabled support for Swift Mailer 6.
• Dropped support Swift Mailer 5.

See #47.

Thanks to @sinamt for this upgrade!

PHP version upgrade

Upgrade php version to ^7.1
Upgrade phpunit version to ^7

Support multipart/related as HTML

See #42 and #43.

0.3.5: Fix attachments always being converted to inline attachments

This is fixing a bug introduced after merging #24

The Content-ID header must not be set for attachments with Content-Disposition: attachment. Postmark is automatically converting attachments with Content-ID to inline attachments, because the Content-Disposition cannot be set in the API.

Test on PHP 7 and update testing infrastructure

• Test on PHP 7
• Test with lowest and highest Composer dependencies
• Other updates to tests and Travis CI configuration
• Add LGTM.co integration for pull requests
• Add EditorConfig
• Update list of Postmark API exceptions

See #30, #36, #37, #38.

Extend Swift_TransportException when throwing Postmark exceptions

This enables you to listed to Postmark API exceptions using the Swift built-in listeners. See #28 for more information.

0.3.2: Add ContentID for inline images

See #24

0.3.1: Dispatch a response event with Postmark API response

See #21.

0.2.1: Dispatch a response event with Postmark API response

See #21.

0.1.7: Dispatch a response event with Postmark API response

See #21.