restrict PHPUnit to v5-v9

PHPUnit 10 fails with old configuration schema while older releases only
print a warning.

Stick with at most v9 for now, as we are testing against older PHP
versions that to not support the new scheme.

composer.json

@@ -1,6 +1,6 @@
 {
   "name": "stklcode/statify-blacklist",
-  "version": "1.6.1",
+  "version": "1.6.2",
   "description": "A filter extension for the famous Statify WordPress plugin",
   "keywords": [
     "wordpress",
@@ -23,10 +23,10 @@
   },
   "require-dev": {
     "dealerdirect/phpcodesniffer-composer-installer": "^0.7",
-    "phpunit/phpunit": "*",
+    "phpunit/phpunit": "^5|^6|^7|^8|^9",
     "phpunit/php-code-coverage": "*",
     "slowprog/composer-copy-file": "~0.3",
-    "squizlabs/php_codesniffer": "^3.6",
+    "squizlabs/php_codesniffer": "^3.7",
     "phpcompatibility/phpcompatibility-wp": "^2.1",
     "wp-coding-standards/wpcs": "^2.3"
   },

package.json

@@ -1,6 +1,6 @@
 {
   "name": "statify-blacklist",
-  "version": "1.6.1",
+  "version": "1.6.2",
   "description": "A filter extension for the famous Statify WordPress plugin",
   "author": "Stefan Kalscheuer",
   "license": "GPL-2.0+"