chore(package): upgrade karma from 3.1.3 to 4.0.0

karma 4.0 dropped node 6 support, which we now also dropped for CI builds.
npm run test (karma tests) runs fine as before.

package.json

@@ -73,7 +73,7 @@
     "html-webpack-plugin": "^3.2.0",
     "http-server": "^0.11.0",
     "jquery": "^3.2.1",
-    "karma": "^3.1.3",
+    "karma": "^4.0.0",
     "karma-base64-to-js-preprocessor": "^0.0.1",
     "karma-chai": "^0.1.0",
     "karma-chrome-launcher": "^2.2.0",