Update AppVeyor PHP 5.6.19 -> 5.6.20

Author: felixfbecker

appveyor.yml

@@ -6,8 +6,8 @@ platform:
 environment:
   matrix:
     - {PHP: 'php-7.0.5-nts-Win32-VC14-x86',  XDEBUG: 'php_xdebug-2.4.0-7.0-vc14-nts'}
-    - {PHP: 'php-5.6.19-nts-Win32-VC11-x86', XDEBUG: 'php_xdebug-2.4.0-5.6-vc11-nts'}
-    - {PHP: 'php-5.6.19-nts-Win32-VC11-x86', XDEBUG: 'php_xdebug-2.3.3-5.6-vc11-nts'}
+    - {PHP: 'php-5.6.20-nts-Win32-VC11-x86', XDEBUG: 'php_xdebug-2.4.0-5.6-vc11-nts'}
+    - {PHP: 'php-5.6.20-nts-Win32-VC11-x86', XDEBUG: 'php_xdebug-2.3.3-5.6-vc11-nts'}
     - {PHP: 'php-5.4.45-nts-Win32-VC9-x86',  XDEBUG: 'php_xdebug-2.4.0-5.4-vc9-nts'}
     - {PHP: 'php-5.4.45-nts-Win32-VC9-x86',  XDEBUG: 'php_xdebug-2.3.3-5.4-vc9-nts'}