Update appveyor URL

See if the updated version can be found and used.

appveyor.yml

@@ -27,7 +27,7 @@ init:
 
 install:
   - cd c:\
-  - appveyor DownloadFile http://windows.php.net/downloads/releases/archives/php-5.6.0-nts-Win32-VC11-x86.zip -FileName php.zip
+  - appveyor DownloadFile http://windows.php.net/downloads/releases/php-5.6.31-nts-Win32-VC11-x86.zip -FileName php.zip
   - 7z x php.zip -oc:\php > nul
   - appveyor DownloadFile https://dl.dropboxusercontent.com/s/euip490d9183jkr/SQLSRV32.cab -FileName sqlsrv.cab
   - 7z x sqlsrv.cab -oc:\php\ext php*_56_nts.dll > nul