composer.json: require-dev phpunit/phpunit update to 7.0

Author: ek9

composer.json

@@ -36,7 +36,7 @@
         "symfony/http-foundation": "^2.3||^3.0||^4.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.0",
+        "phpunit/phpunit": "^7.0",
         "mikey179/vfsStream": "1.6.*",
         "squizlabs/php_codesniffer": "^3.2",
         "escapestudios/symfony2-coding-standard": "^3.1"