[skip travis] AppVeyor: use npm 2.15.9 for node > 0.x to work around …

…"'__pfnDliNotifyHook2': redefinition;"

scripts/build-appveyor.bat

@@ -50,7 +50,10 @@ powershell Set-ExecutionPolicy Unrestricted -Scope CurrentUser -Force
 IF %ERRORLEVEL% NEQ 0 GOTO ERROR
 CALL npm install --global --production npm-windows-upgrade
 IF %ERRORLEVEL% NEQ 0 GOTO ERROR
-CALL npm-windows-upgrade --npm-version 2.15.6 --no-dns-check --no-prompt
+REM https://ci.appveyor.com/project/Mapbox/node-sqlite3/build/1.0.500/job/n2y9fo4eg316db56#L289
+REM error C2373: '__pfnDliNotifyHook2': redefinition; different type modifiers
+REM at least 2.15.9 is needed: https://github.com/nodejs/node-gyp/issues/972#issuecomment-231055109
+CALL npm-windows-upgrade --npm-version 2.15.9 --no-dns-check --no-prompt
 IF %ERRORLEVEL% NEQ 0 GOTO ERROR
 
 :SKIP_APPVEYOR_INSTALL