There might be issues with running npm under PS,
therefore let's run npm under CMD.

https://github.com/sass/node-sass/issues/996

http://help.appveyor.com/discussions/problems/2347-npm-275prefer-global-warning-causes-failure-command-executed-with-exception#comment_37223989

https://github.com/npm/npm/issues/8517

Effectively reverts: https://github.com/sass/node-sass/pull/995