fix accidentally removed msvs_version=2013 support (for node v4)

Author: springmeyer

scripts/build-appveyor.bat

@@ -8,6 +8,7 @@ IF /I "%msvs_toolset%"=="" ECHO msvs_toolset unset, defaulting to 14 && SET msvs
 IF /I "%msvs_version%"=="" ECHO msvs_version unset, defaulting to 2015 && SET msvs_version=2015
 
 SET PATH=%CD%;%PATH%
+IF "%msvs_toolset%"=="12" SET msvs_version=2013
 IF NOT "%NODE_RUNTIME%"=="" SET "TOOLSET_ARGS=%TOOLSET_ARGS% --runtime=%NODE_RUNTIME%"
 IF NOT "%NODE_RUNTIME_VERSION%"=="" SET "TOOLSET_ARGS=%TOOLSET_ARGS% --target=%NODE_RUNTIME_VERSION%"