bump php to 7.3 & phpunit to 9.6 should fix #17

Author: kratzky

composer.json

@@ -31,13 +31,13 @@
         }
     ],
     "require": {
-        "php": ">=5.6",
+        "php": ">=7.3",
         "ext-curl": "*",
         "ext-mbstring": "*",
         "ext-fileinfo": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "8.5.*"
+        "phpunit/phpunit": "9.6.*"
     },
     "autoload": {
         "psr-4": {