Allow recent versions of PHPUnit

Author: greg0ire

.gitignore

@@ -1,4 +1,5 @@
 phpunit.xml
+/.phpunit.result.cache
 build
 vendor
 /composer.lock

composer.json

@@ -25,7 +25,7 @@
         "phpbench/phpbench":         "^0.13",
         "phpstan/phpstan":           "^0.12",
         "phpstan/phpstan-phpunit":   "^0.12",
-        "phpunit/phpunit":           "^7.0"
+        "phpunit/phpunit":           "^7.0 || ^8.0 || ^9.0"
     },
     "autoload": {
         "psr-4": {