Use APPVEYOR_BUILD_VERSION instead of the nonexistent VERSION variable

appveyor.yml

@@ -19,6 +19,6 @@ configuration:
 before_build:
 - nuget restore
 after_build:
-- makensis /DSSH_HANDLER_VERSION=%VERSION% /DSSH_HANDLER_CONFIGURATION=%CONFIGURATION% ssh-handler.nsi
+- makensis /DSSH_HANDLER_VERSION=%APPVEYOR_BUILD_VERSION% /DSSH_HANDLER_CONFIGURATION=%CONFIGURATION% ssh-handler.nsi
 cache:
 - dotNetFx40_Client_setup.exe