PHP 8.0 support

[gpgkd906]

Signed-off-by: chenhan gpgkd906@gmail.com

Q	A
New Feature	yes

PHP 8.0 support #6

Description

• Modify composer.json to provide support for PHP 8.0 by adding the constraint ~8.0.0
• Modify composer.json to drop support for PHP less than 7.3
• Modify composer.json to implement phpunit 9.3 which supports PHP 7.3+
• Modify .travis.yml to ignore platform requirements when installing composer dependencies (simply add --ignore-platform-reqs to COMPOSER_ARGS env variable)
• Modify .travis.yml to add PHP 8.0 to the matrix (NOTE: Do not allow failures as PHP 8.0 has a feature freeze since 2020-08-04!)
• Modify source code in case there are incompatibilities with PHP 8.0

[gpgkd906]

re-structure commits