Support for Laravel 5.2

Laravel 5.2 eliminated "pretend" and "log" features from the base mailer, which caused the postmark provider to break. We're updating to get this working. Since this is technically a breaking change, we're bumping to 3.0, but there are minimal changes to the provider.

Updates guzzle to 6.0

This release supports newer versions of Laravel by updating the Guzzle requirement to 6.0+

1.0.0

Initial release of package.

This release simply corrects the dependency graph created for working with the swiftmailer transport, and Laravel. No modifications to the Laravel Adapter were made upon separating it from the wildbit/swiftmailer-postmark package.

This was done due to the suggestion from @mlebkowski in this issue: ActiveCampaign/swiftmailer-postmark#1