Use Visual Studio 2015 for Node 6+
Use Visual Studio 2015 for Node 6+

Node change to Visual Studio 2015 when Node 6 went to LTS.

See nodejs/node#7989
Fixes #1854

xzyfer/node-sass,xzyfer/node-sass,nschonni/node-sass,saper/node-sass,sass/node-sass,sass/node-sass,xzyfer/node-sass,xzyfer/node-sass,saper/node-sass,nschonni/node-sass,saper/node-sass,xzyfer/node-sass,saper/node-sass,Smartbank/node-sass,Smartbank/node-sass,nschonni/node-sass,nschonni/node-sass,Smartbank/node-sass,sass/node-sass,xzyfer/node-sass,Smartbank/node-sass,saper/node-sass,sass/node-sass,Smartbank/node-sass,nschonni/node-sass,sass/node-sass,sass/node-sass,saper/node-sass,nschonni/node-sass,xzyfer/node-sass,nschonni/node-sass,Smartbank/node-sass,sass/node-sass,Smartbank/node-sass,saper/node-sass