chore(ci): use cmd instead of powershell

Appveyor testing is failing due to a warning during installation of the
`serverless` package, since the `fsevents` dependency is targeting the
`darwin` architecture.

Signed-off-by: Mike Fiedler <miketheman@gmail.com>

Author: miketheman

appveyor.yml

@@ -3,7 +3,7 @@ init:
     - cmd: python -m pip install -U pip
     - cmd: pip install pipenv
     - cmd: pip install poetry==1.0.0a2
-    - ps: npm i -g serverless
+    - cmd: npm i -g serverless
 build: off
 test_script:
     - cmd: >-