Lighter AppVeyor config

[jdesboeufs]

Hi,

Currently we have 16 builds in AppVeyor:

• 4 Node versions
• x 2 architectures
• x 2 Visual Studio toolsets

If AppVeyor works well, it takes more than 2 hours 40 minutes to complete a build sequence, which is a bit too much.

As we plan to drop Node 0.10 support, we fall to 12 build and 2 hours.

Can we also drop the Visual Studio 2013 toolset, since we are now on modern Node versions? We would have finally 6 builds = ~1 hour.

We could also go further and just build minimal environments for PR, and full sequence for releases.

WDYT?