Update Travis CI config

[fnogatz]

• Remove unnecessary environment variable GITHUB_OAUTH_TOKEN (former API limit has been removed in 2016)
• Remove manual doctrine/instantiator downgrade
• We support only PHP 7.3 and 7.4 (former versions reached EOL)
• We support only Magento 2 versions starting from v2.3.5
• Add declaration of return value in setUp(): void in *Test.php as required by PHPUnit

Pull request resolves #52.