Update phpunit/phpunit requirement from ^6.0 || ^7.0 to ^7.0 || ^8.0 (#43)

Author: dependabot[bot]

README.md

@@ -1,5 +1,5 @@
 # Symfony JSON-RPC server
-[![License](https://img.shields.io/github/license/yoanm/symfony-jsonrpc-http-server.svg)](https://github.com/yoanm/symfony-jsonrpc-http-server) [![Code size](https://img.shields.io/github/languages/code-size/yoanm/symfony-jsonrpc-http-server.svg)](https://github.com/yoanm/symfony-jsonrpc-http-server) [![Dependencies](https://img.shields.io/librariesio/github/yoanm/symfony-jsonrpc-http-server.svg)](https://libraries.io/packagist/yoanm%2Fsymfony-jsonrpc-http-server)
+[![License](https://img.shields.io/github/license/yoanm/symfony-jsonrpc-http-server.svg)](https://github.com/yoanm/symfony-jsonrpc-http-server) [![Code size](https://img.shields.io/github/languages/code-size/yoanm/symfony-jsonrpc-http-server.svg)](https://github.com/yoanm/symfony-jsonrpc-http-server) [![Dependabot Status](https://api.dependabot.com/badges/status?host=github&repo=yoanm/symfony-jsonrpc-http-server)](https://dependabot.com)
 
 [![Scrutinizer Build Status](https://img.shields.io/scrutinizer/build/g/yoanm/symfony-jsonrpc-http-server.svg?label=Scrutinizer&logo=scrutinizer)](https://scrutinizer-ci.com/g/yoanm/symfony-jsonrpc-http-server/build-status/master) [![Scrutinizer Code Quality](https://img.shields.io/scrutinizer/g/yoanm/symfony-jsonrpc-http-server/master.svg?logo=scrutinizer)](https://scrutinizer-ci.com/g/yoanm/symfony-jsonrpc-http-server/?branch=master) [![Code Coverage](https://img.shields.io/scrutinizer/coverage/g/yoanm/symfony-jsonrpc-http-server/master.svg?logo=scrutinizer)](https://scrutinizer-ci.com/g/yoanm/symfony-jsonrpc-http-server/?branch=master)
 

composer.json

@@ -41,7 +41,7 @@
   "require-dev": {
     "behat/behat": "~3.0",
     "squizlabs/php_codesniffer": "3.*",
-    "phpunit/phpunit": "^6.0 || ^7.0",
+    "phpunit/phpunit": "^7.0 || ^8.0",
     "matthiasnoback/symfony-dependency-injection-test": "^2.0 || ^3.0",
     "matthiasnoback/symfony-config-test": "^3.0 || ^4.0",
     "symfony/framework-bundle": "^3.0 || ^4.0",