change dev-requirement phpunit version for support PHP 7.0 - 8.0 versions

Author: ddrv

composer.json

@@ -19,7 +19,7 @@
   },
   "require-dev": {
     "nyholm/psr7": "^1.3",
-    "phpunit/phpunit": "^6.5",
+    "phpunit/phpunit": ">=6.5",
     "squizlabs/php_codesniffer": "^3.5",
     "webclient/fake-http-client": "^1.0"
   },

phpunit.xml.dist

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit bootstrap="vendor/autoload.php" colors="true">
+<phpunit bootstrap="vendor/autoload.php" colors="true" cacheResult="false">
     <testsuites>
         <testsuite name="all">
             <directory>tests</directory>