make YourlsProxyTest compatible with newer phpunit

tst/YourlsProxyTest.php

@@ -12,7 +12,7 @@ class YourlsProxyTest extends TestCase
 
     private $_mock_yourls_service;
 
-    public function setUp()
+    public function setUp(): void
     {
         /* Setup Routine */
         $this->_path = sys_get_temp_dir() . DIRECTORY_SEPARATOR . 'privatebin_data';
@@ -29,7 +29,7 @@ class YourlsProxyTest extends TestCase
         $this->_conf = new Configuration;
     }
 
-    public function tearDown()
+    public function tearDown(): void
     {
         /* Tear Down Routine */
         unlink(CONF);