Feature / Fix: Support PHP 7.4 by skipping unneeded cleanup of exception trace args. (#92 by @clue)

Clean up test suite and add .gitattributes to exclude dev files from exports. Run tests on PHP 7.4, PHPUnit 9 and simplify test matrix. Link to using SSH proxy (SSH tunnel) as an alternative. (#88 by @clue and #91 and #93 by @SimonFrings)