Adding missing npm dep

Author: Lucas-C

.github/workflows/ci.yml

@@ -66,7 +66,7 @@ jobs:
 
         - name: Node.js 5.x with core-js polyfill
           node-version: "5.12"
-          npm-i: browserify@15 mocha@5.2.0 nyc@11.9.0
+          npm-i: browserify@15 mocha@5.2.0 nyc@11.9.0 core-js@3
           ENABLE_COREJS_ERROR_POLYFILL: true
 
         - name: Node.js 6.x