[VENDOR] Upgrade phpunit to ^6.5 for code coverage support

Author: Trungin Ton

composer.json

@@ -32,7 +32,8 @@
         "learnosity/learnosity-sdk-php": "^0.7.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.7"
+        "phpunit/php-code-coverage": "^5.2",
+        "phpunit/phpunit": "^6.0"
     },
     "bin": [
         "bin/mo"