Update AppVeyor config with ~faster npm install

[JaKXz]

Only test against stable and LTS versions of node since AppVeyor goes pretty slow.

@bcoe I noticed that after dropping the Travis build times to 1/4th of what they were on Travis in #251, AppVeyor's build takes even longer... so this is an attempt to streamline it by using the version of npm that comes with each node distro and only going for stable and LTS versions. Thoughts?

[bcoe]

@JaKXz this seems to be failing, kicked off the build twice. Perhaps try adding back the npm update, and see if this helps; if not, we might not be able to pull in 6.x yet.

[JaKXz]

@bcoe I don't think the npm update is what's broken here, since node@6 comes with a version that I'm pretty sure npm install -g npm@latest would install [at time of writing] - see the AppVeyor logs around line 507:

node --version
v6.1.0
npm --version
3.8.6

Though I'm not sure why it would pass on Travis and not AppVeyor with node@6.

[bcoe]

@JaKXz I think we'll need to spin up a Windows VM to test this one out.

[jamestalmage]

Probably related to:
avajs/ava#814

[JaKXz]

whoa it's passing now... 🎱

[jamestalmage]

Yes. Node 6.2.0 reverted a regression that almost certainly affected this.

[jamestalmage]

0.12 remains in LTS mode through the end of the year. We should not drop it yet.

[bcoe]

@JaKXz interesting that it started working, maybe the version of 6 was updated.