Mailgun PHP SDK v3.2.0

Introducing GitHub Actions as part of the CI process for this repository.

This release has changed the CI process from Travis CI to GitHub Actions. The reasons for this decision are:

• For some reason, Travis CI queues the job but takes ages before it starts running it
• Travis CI doesn't provide (yet) the support we need to start testing for PHP 8.x

Changelog

Added

• Domain tracking implementation (#711 by @uavn)
• Mailing list validation (#712 by @uavn)
• Suppression Whitelists (#713 by @uavn)

Fixed

• Added pagination to tags (#717 by @uavn)

Changed

• Expect Client to be of type ClientInterface or PluginClient (#660 by @tonythomas01)