chore(appveyor): do not use latest npm

[nfischer]

This changes appveyor from using the latest npm version to only using
the preinstalled npm version. This is safer, because it's guaranteed to
be a compatible version.

Fixes #844

[codecov-io]

Codecov Report

Merging #847 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #847   +/-   ##
======================================
  Coverage    95.8%   95.8%           
======================================
  Files          34      34           
  Lines        1262    1262           
======================================
  Hits         1209    1209           
  Misses         53      53

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 97a4df8...6542ebd. Read the comment docs.

[freitagbr]

LGTM

[nfischer]

I'm landing this, because we need to fix master ASAP. However, there's a caveat to this change: npm recommends npm@3 on Windows, however this change bumps the node4 bot down to npm@2.15.11.

It's not clear if this causes correctness issues. This may merit further investigation.