Upgrade ruby action (#62)

The actions/setup-ruby action is deprecated
This commit updates the CI job to use the new ruby setup action

Author: ConnorKirk

.github/workflows/unit-tests.yaml

@@ -15,7 +15,7 @@ jobs:
     name: Unit tests
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/setup-ruby@v1
+      - uses: ruby/setup-ruby@v1
         with:
           ruby-version: '2.6'
       - run: gem install cfn-nag