build: Update BUILD.md to specify VS2015
build: Update BUILD.md to specify VS2015

Change-Id: I9180304a8089db057af68cbf86b73118e1d29c16

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools