Spectral 5.0.0 on macOS

Question , using spectral on macOS

I installed the binary for spectral on MacOS Mojave.
spectral --version
5.0.0
npm --version
6.13.4

I was able to successfully run Spectral against a local ruleset file,
e…g
spectral lint swagger.json --ruleset=/Users/adrian/Downloads/spectral2.yml

OpenAPI 2.0 (Swagger) detected

/Users/adrian/onap_nbi_latest/nbi/docs/offeredapis/api_hub/swagger.json

182:12 warning model-definitions-examples Model property examples SHOULD be present and non-empty string.

but when I add the line:
extends: spectral:oas2

as the first line to my local ruleset I get the error below

spectral lint swagger.json --ruleset=/Users/adrian/Downloads/spectral2.yml
Could not parse https://unpkg.com/spectral:oas2: Forbidden

Any pointers on what I am doing wrong greatly appreciated.