Update PHPUnit to v9

This is a big jump for due to the lack of tests it's an easy fix.
See the following links for all changes:
- https://phpunit.de/announcements/phpunit-9.html
- https://phpunit.de/announcements/phpunit-8.html
- https://phpunit.de/announcements/phpunit-7.html
- https://phpunit.de/announcements/phpunit-6.html
- https://phpunit.de/announcements/phpunit-5.html

composer.json

@@ -30,7 +30,7 @@
         "illuminate/pagination": ">=5.0.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "~4.0"
+        "phpunit/phpunit": "~9.0"
     },
     "autoload": {
         "psr-4": {

tests/InitialTest.php

@@ -16,11 +16,11 @@
 class InitialTest extends TestCase {
     protected $cm;
 
-    public function setUp() {
+    public function setUp(): void {
         $this->cm = new ClientManager();
     }
 
     public function testConfigDefaultAccount() {
         $this->assertEquals("default", ClientManager::get("default"));
     }
-}
+}