wildbit/postmark-php 4.0.0

Add support for Guzzle 7 and PHP 7.1-7.4

on PHP Packagist

Changes:

• Add support for Guzzle 7 (we keep supporting Guzzle 6 as well) (#73)
• Add support for PHP 7.1 - 7.4 (#73)
• Drop support for PHP 5.5 and 5.6

Minor fixes:

• Add composer.lock to .gitignore
• Cleanup .travis.yml and switch to xenial dist
• Update PHPUnit to ^6.0 and fix risky tests

Big thanks to @svycka for sending the PR!