Extend allowed PHPUnit versions.

Author: RoSk0

composer.json

@@ -25,7 +25,7 @@
     "simplesamlphp/composer-module-installer": "~1.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "^5|^6",
+    "phpunit/phpunit": "^5|^6|^7",
     "squizlabs/php_codesniffer": "^2.0.0|^3.0.0"
   },
   "autoload-dev": {