Use a different version of signtool.

Author: Dirk Lemstra

appveyor.yml

@@ -16,7 +16,7 @@ environment:
     secure: bQc3Nboe/KLQhUg19FYGyNxL+w6OQ7pTTE2OXPBi2JD9aQICOH9ZNO8MRioScKRSeMR39Jw1T/s3Pnz5pZ2K4pbntOqS5FnF94VnKkekee9MYHOnhSOMh4YrI9D+HmRdTuC+Q0B6IsO2gC1iATI7y2RkbB7cOHYgycyGVuotLVs=
   KeyFile: C:\ImageMagick.pfx
 
-  SignTool: C:\Program Files (x86)\Microsoft SDKs\Windows\v7.1A\Bin\signtool.exe
+  SignTool: C:\Program Files (x86)\Windows Kits\8.1\bin\x64\signtool.exe
   CertPassword:
     secure: FPTUZszs2LsBXvLLO0u7rD/UJaT2IbyNZAxL9Kjhq4VSZB/hTDq87E7qNN+Wfj+F