fix node-gyp version to use released version

Author: Dane Springmeyer

scripts/build-appveyor.bat

@@ -112,7 +112,7 @@ ECHO ===== where npm puts stuff END ============
 
 IF "%nodejs_version:~0,1%"=="0" npm install https://github.com/springmeyer/node-gyp/tarball/v3.x
 IF %ERRORLEVEL% NEQ 0 GOTO ERROR
-IF "%nodejs_version:~0,1%"=="4" npm install node-gyp@4.x
+IF "%nodejs_version:~0,1%"=="4" npm install node-gyp@3.x
 IF %ERRORLEVEL% NEQ 0 GOTO ERROR
 
 CALL npm install --build-from-source --msvs_version=%msvs_version% %TOOLSET_ARGS% --loglevel=http