fix: don't add rules if they're not available

Author: MichaelDeBoey

.github/workflows/CI.yml

@@ -16,6 +16,9 @@ on:
 jobs:
     lint:
         name: ⬣ Lint
+        strategy:
+            matrix:
+                eslint: [6, 7]
         runs-on: ubuntu-latest
         steps:
             - name: 🛑 Cancel Previous Runs
@@ -32,6 +35,9 @@ jobs:
             - name: 📥 Install dependencies
               run: npm install
 
+            - name: 📥 Install ESLint v${{ matrix.eslint }}
+              run: npm install --save-dev eslint@${{ matrix.eslint }}
+
             - name: ▶️ Run lint script
               run: npm run lint