Fix appveyor

[mapsam]

Appveyor is busted, let's fix it! (or get rid of it)

It looks like we are installing our own versions of node from the mapbox CDN. Curious if we could move over to testing out with Appveyor's built-in versions so we can be a bit more flexible with updating to v4/v6.

I assume we are building on Visual Studio 2015 so @BergWerkGIS is able to test locally. Some insight from you would be great here!

cc @springmeyer @GretaCB @flippmoke @artemp

[springmeyer]

Curious if we could move over to testing out with Appveyor's built-in versions so we can be a bit more flexible with updating to v4/v6.

Not currently. The node-mapnik binaries depend on a custom node compiled with visual studio 2015. So #166 (comment) still stands.

I assume we are building on Visual Studio 2015 so @BergWerkGIS is able to test locally.

We build with vs 2015 since It is the only runtime to support C++11 per https://github.com/mapbox/node-cpp11. @BergWerkGIS and I are working to figure out a way forward (to be able to support node v4/6, but we're not there yet).