Update caching library to latest version, updated and fixed phpUnit tests

tests/OfflineTests/AccountGateway/AccountContextTest.php

@@ -8,11 +8,11 @@ namespace CloudObjects\SDK\AccountGateway;
 
 use ML\IRI\IRI;
 
-class AccountContextTest extends \PHPUnit_Framework_TestCase {
+class AccountContextTest extends \PHPUnit\Framework\TestCase {
 
     private $context;
 
-    protected function setUp() {
+    protected function setUp(): void {
         $this->context = new AccountContext(new IRI('aauid:aaaabbbbccccdddd'), 'DUMMY');
     }