fix: effective_severity attribute error

[askmeegs]

Fixes #47

How to test the sample -

1. clone repo
2. go to a GCP project > Container Registry > Settings > Enable Vulnerability scanning
3. push the ubuntu bionic image to GCR in your project (see docs)
4. set up virtualenv + install packages, making sure GOOGLE_APPLICATION_CREDENTIALS env var is set for your project
5. run the sample function where project_id is your project ID, and resource URL is this:

https://gcr.io/{project_id}/ubuntu@sha256:{sha_hash}

You can get the sha_hash by going to container registry > your image > clicking "copy full image name," and you should get something like this -

gcr.io/megan-fall20/ubuntu@sha256:32776cc92b5810ce72e77aca1d949de1f348e1d281d3f00ebcc22a3adcdc9f42

Where 327... is the sha_hash.

[Shabirmean]

LGTM!