Allow us to use this package on newer applications

[antonioribeiro]

I wasn't able to install this package on a Laravel 7 application and I believe anyone relying on Guzzle 6.5 wouldn't too.

This PR allows newer Guzzle versions to be used:

"guzzlehttp/guzzle": "~5|~6"

But I also updated the test suite to use newer PHPUnit (from version 5 to 9). Unfortunately PHP 7.0 hit end-of-life some some time ago, but PHP 7.1 (which also hit EOL) is still being supported by tests.

[msftclas]

All CLA requirements met.