update to phpunit8

Behavioral/ChainOfResponsibilities/Tests/ChainTest.php

@@ -15,7 +15,7 @@ class ChainTest extends TestCase
      */
     private $chain;
 
-    protected function setUp()
+    protected function setUp(): void
     {
         $this->chain = new HttpInMemoryCacheHandler(
             ['/foo/bar?index=1' => 'Hello In Memory!'],

Behavioral/Visitor/Tests/VisitorTest.php

@@ -13,7 +13,7 @@ class VisitorTest extends TestCase
      */
     private $visitor;
 
-    protected function setUp()
+    protected function setUp(): void
     {
         $this->visitor = new Visitor\RoleVisitor();
     }