Update PHPUnit version

Author: byjg

composer.json

@@ -19,7 +19,7 @@
         "byjg/jwt-wrapper": "2.0.*"
     },
     "require-dev": {
-        "phpunit/phpunit": ">=5.7"
+        "phpunit/phpunit": "5.7.*|7.4.*"
     },
     "license": "MIT"
 }