change dev-requirement phpunit version for php 7.0-8.0 support

Author: ddrv

composer.json

@@ -18,7 +18,7 @@
   },
   "require-dev": {
     "guzzlehttp/psr7": "^1.7",
-    "phpunit/phpunit": "^6.5",
+    "phpunit/phpunit": ">=6.5",
     "squizlabs/php_codesniffer": "^3.5",
     "webclient/fake-http-client": "^1.0"
   },