Allow PHP 8, fix tests and README, update dev dependencies

Author: amaccis

README.md

@@ -1,6 +1,6 @@
 # php-stemmer
 
-[![PHP Version](https://img.shields.io/badge/php-%5E7.4-blue.svg)](https://img.shields.io/badge/php-%5E7.4-blue.svg)
+[![PHP Version](https://img.shields.io/badge/php-%5E7.4%7C%5E8.0-blue.svg)](https://img.shields.io/badge/php-%5E7.4%7C%5E8.0-blue.svg)
 ![CI](https://github.com/amaccis/php-stemmer/workflows/CI/badge.svg)
 
 ## What is PHP Stemmer?

composer.json

@@ -14,11 +14,11 @@
         }
     },
     "require": {
-        "php": "^7.4",
+        "php": "^7.4 || ^8.0",
         "ext-ffi": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^8",
-        "phpstan/phpstan": "^0.12.11"
+        "phpunit/phpunit": "^9.5",
+        "phpstan/phpstan": "^0.12"
     }
 }