use CALL

Author: Dane Springmeyer

scripts/build-appveyor.bat

@@ -115,11 +115,11 @@ IF /I "%NPM_BIN_DIR%"=="%CD%" ECHO ERROR npm bin -g equals local directory && SE
 ECHO ===== where npm puts stuff END ============
 
 
-IF "%nodejs_version:~0,1%"=="0" npm install https://github.com/springmeyer/node-gyp/tarball/v3.x
+IF "%nodejs_version:~0,1%"=="0" CALL 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@3.x
+IF "%nodejs_version:~0,1%"=="4" CALL npm install node-gyp@3.x
 IF %ERRORLEVEL% NEQ 0 GOTO ERROR
-IF "%nodejs_version:~0,1%"=="5" npm install node-gyp@3.x
+IF "%nodejs_version:~0,1%"=="5" CALL 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