Add --vs2015 to windows setup instructions

CONTRIBUTING.md

@@ -40,7 +40,7 @@ Then you need `git`, if you don't have that yet: https://git-scm.com/
       https://www.microsoft.com/en-us/download/details.aspx?id=40773
     * Install Windows SDK version 8.1: https://developer.microsoft.com/en-us/windows/downloads/sdk-archive
 1.  Install _Windows Build Tools_: Open the [Command Prompt (`cmd.exe`) as Administrator](<https://technet.microsoft.com/en-us/library/cc947813(v=ws.10).aspx>)
-    and run: `npm install --global --production --add-python-to-path windows-build-tools`
+    and run: `npm install --vs2015 --global --production --add-python-to-path windows-build-tools`
 
 ### Linux