Set phpunit as require-dev dependency

Author: pitbulk

composer.json

@@ -16,10 +16,10 @@
     },
     "require": {
         "php": ">=5.4",
-        "robrichards/xmlseclibs": ">=3.1.1",
-        "phpunit/phpunit": "<7.5.18"
+        "robrichards/xmlseclibs": ">=3.1.1"
     },
     "require-dev": {
+        "phpunit/phpunit": "<7.5.18",
         "php-coveralls/php-coveralls": "^1.0.2 || ^2.0",
         "sebastian/phpcpd": "^2.0 || ^3.0 || ^4.0",
         "phploc/phploc": "^2.1 || ^3.0 || ^4.0",