Add direct refund API

• add direct refund API function
• add response getter on Notification class

Compatibility fix for PHP v7.4

Ensure full compatibility with latest stable version of PHP v7.4

• Replace deprecated array_key_exist() on object of PHP 7.4
• Add more PHP version to Travis CI test

Minor update

• update outdated test keys

Initial v2.0

• Renamed to Midtrans
• Namespace compatible with composer standard
• Basic CoreAPI & Snap API functionality
• Published to Packagist