AppVeyor: Pin VS version to 2013 for the time being
AppVeyor: Pin VS version to 2013 for the time being

With the new oss environments, AppVeyor images have many Visual Studio
versions installed at once. CMake will pick the highest, but we don't
have Boost binaries compiled for VS 2015. Stick with VS 2013.

Soon, the VS version will be chosen using a variable in the appveyor.yml file.

ichramm/LuaNode,ignacio/LuaNode,ignacio/LuaNode,ichramm/LuaNode,ichramm/LuaNode,ichramm/LuaNode,ignacio/LuaNode,ignacio/LuaNode