v1.6.0

• #13 Default adapters without dependencies (@Zales0123)
• #19 [Documentation] Extracted from Sylius/Sylius repository ()
• #24 Switch from Travis to GitHub Actions (@pamil)
• #25 Bump requirements to PHP ^7.3 and Symfony ^4.4|^5.1 (@pamil)
• #26 Normalize composer.json files (@pamil)
• #27 Add Psalm v4 (@pamil)
• #28 Add support for PHP 8 (@pamil)
• #29 Minor updates for licenses, changelogs and readmes (@pamil)