AppVeyor: update WinSCP download link

Goodbye, SourceForge!

appveyor.yml

@@ -40,8 +40,8 @@ after_build:
             7z a $BUILD_NAME_NOQT .\build\bin\release\*.exe
 
             # Download winscp
-            Invoke-WebRequest "http://hivelocity.dl.sourceforge.net/project/winscp/WinSCP/5.7/winscp570.zip" -OutFile "winscp570.zip"
-            7z e -y winscp570.zip
+            Invoke-WebRequest "http://winscp.net/download/winscp573.zip" -OutFile "winscp573.zip"
+            7z e -y winscp573.zip
 
             # Upload to server
             .\WinSCP.com /command `