Add support for Laravel 8.0 with support for guzzle 7.0.*

[sebastiansson]

Description

Add support for Laravel 8.0 with support for guzzle 7.0.*.

Checklist

• [x] Code compiles correctly
• [x] Code conforms PSR-2. Recommending use PHP CS Fixer with next rules: --rules=@PSR2,blank_line_before_return,no_unused_imports
• [x] Created tests which fail without the change (if possible)
• [x] All tests passing
• [x] Extended the README, if necessary
• [x] Added myself / the copyright holder to the authors in composer.json file