build: drop support for VS 2013 in v6 LTS #7989

This is split off from #7484 to discuss the possibility of dropping VS 2013 in v6 LTS. From the other issue:

Proposal: drop support for building node.js with VS 2013.

Motivation: VS 2013 has numerous bugs in its C++11 support and in general. The source tree is full of hacks that work around this broken compiler and pull requests sometimes strand on it (e.g. #5458.)

As to add-ons, we could extend VS 2013 support for our public headers for a while (and test that through test/addons) but V8 will probably force us to update the baseline there too. Chromium currently requires Visual Studio 2015 Update 2 so I expect V8 will follow suit sooner rather than later.

@joaocgreis reported in #7484 (comment):

I tested some of the most downloaded native modules (ws (to test bufferutil and utf-8-validate), node-sass, contextify, thread-sleep, weak, bcrypt, hiredis, sqlite3, v8-profiler, iconv, ref and v8-debug) using VS2013 and they all behave exactly the same whether node itself was built with VS2013 or VS2015.

So I think we should go ahead with this and start building v6 and v7 with VS2015.

Encouraging!